May 152014
 
In theory, the 55 panels on this roof might be capable of providing up to 14 kW of power.  But, in reality?  Probably closer to 10 kW.

In theory, the 55 panels on this roof might be capable of providing up to 14 kW of power. But, in reality? Probably closer to 10 kW.

So there you are, thinking about buying some solar panels.  You’ve noticed they come in a range of semi-standard shapes and sizes, and maybe you’ve even noticed that the slightly unusual dimensions (such as 39.4″) actually make sense if converted to metrics (ie 39.4″ = 1 meter).

You also notice all sorts of acronyms floating around in the specifications and warranties, and you sort of wonder if, when comparing the power output of Brand X’s 250 watt panel with that of Brand Y’s 255 watt panel, if there truly is 5 watts of difference, and, for that matter, whether either panel will ever produce close to the claimed 250 watt output, and what that actually means in terms of total kWhrs a day.

Perhaps even worse are advertisements with no acronyms or qualifiers at all, just a list of unexplained specifications.  Who is making those claims, and how credible are they?

These are all good questions.  We’ll try to answer them for you.

The good news is that there are some official standards that can apply to how solar panel power outputs are measured.  The not so good news is that while these official standards might provide a level playing field for how to measure one panel’s power output alongside another panel, the results obtained by the standards do not necessarily match the real world experience you’ll get (a bit like how the official mpg figures for new cars are seldom the same as you get yourself in real-world driving).  But first, let’s understand exactly what solar panels give you, and why it so quickly becomes difficult to establish their true power output.

All solar panels provide their power in DC volts and amps.  The actual power they provide (which is measured in watts) is calculated by multiplying their output voltage by the amps of current that flows at that voltage – this might seem like a simple calculation, but it isn’t – the voltage level varies based on the amps that are flowing, and both also vary based on the intensity of the sunlight falling on the panel.  So even a simple seeming power measurement isn’t quite as simple as it should be.

It gets worse.  When you start connecting a series of panels together, the real world practical power you might get is not necessarily the simple sum of the power outputs of each individual panel.

However, simple or not, a DC watt specification is the most direct measure of their power output.  Occasionally you may see panels with an AC wattage rating – these would be panels with individual ‘micro-inverters’ that convert the DC output of the panel immediately to AC, right at the panel.

At least until recently, it has been most common to connect together the DC output from multiple panels, then feed that combined power to a single central inverter that then converts it to AC.  But there are convincing studies to suggest that micro-inverters are a very good thing, and while they might slightly add to the cost of a solar array installation, they might also result in you getting appreciably more power out of the system in real life, as compared to the implied power outputs quoted by the specifications.

For now, simply be aware that all inverters involve a slight and inevitable power loss (typically an inverter is anywhere from 95% to 98% efficient) and so if you are seeing an AC watt rating, this has already had the inverter power loss removed.  For example, a 250W DC panel, after passing through a 96% efficient inverter, would end up giving you 240W of AC power.

In other words, AC watts are generally ‘better’ than DC watts, when comparing numbers.

Now for some official ‘standards’ for solar cell power measurements, and note that usually power measurements are made by the manufacturer, rather than by an independent third-party, so there is a certain amount of trust required when accepting these numbers, no matter what the standard may be that they are claimed to have been measured by.

Many cells are rated based on a STC rating.  STC stands for ‘Standard Test Conditions’.  These are an ambient temperature of 25°C/77°F, sunlight of a 1000 W/sq m intensity falling directly on the panel, an air mass of 1.5, and zero wind speed.

Another rating is the NOCT rating.  This is the Normal Operating Cell Temperature rating, and it will always give a lower rating.  NOCT ratings assume 800 W/sq m of solar power falling on the cell, a 20°C/68°F ambient temperature, and a wind of 1 m/sec (2.24 mph) blowing on the backside of the solar panel for cooling.

Even this is optimistic.  The way most solar panels are laid out prevents any underneath cooling, and so their temperatures can rise appreciably, and as they get hotter, they become less efficient (once the air temperature gets up into the high 80s, you’re probably going to start losing 1% of power for every two degrees F of temperature rise).

But wait – there’s more.  Would you be surprised to learn that California does its own thing?  It uses a different standard, the PTC standard.  Unlike the STC rating, the PTC rating is not a measured rating, but a theoretical rating.  That might seem like a backwards step, but it is based on adjusted realworld data, and unlike the self-assessed STC rating, the PTC rating, at least as expressed by California’s CEC (California Energy Commission) requires independent lab results rather than accepting manufacturer claims.

PTC stands for Photovoltaics for Utility Systems Applications Test Conditions, in case you wondered.

Here is an interesting table of PTC ratings for solar panels.  If you go down the list, you’ll see that sometimes panels with a manufacturer stated lower power capacity than another panel actually test as giving more power, and you’ll see appreciable differences between panels all offering apparently the same output.

So maybe you can decide that your 250 W panel actually produces 225 watts to start with.

But then, you need to start adjusting further down.  Perhaps the panel was slightly under specs when it came from the manufacturer.  It will probably lose almost 1% of its output each year that it is operational – do you want to plan your system based on its best case output when brand new, or its mid-life output when it is 5, 10, even 20 years old?

The allow for inverter losses, additional losses through its wiring, and some shading/uneven lighting losses (both from clouds as well as from things like trees around your site).  Add a bit more for other miscellaneous electrical losses, and some for dirt on the panels, and all of a sudden, that 250W panel is starting to promise you more like 150 watts of real power.

One of the subtle but potential huge power losses is from shading.  Now you sort of understand that if the sunlight halves in ‘strength’, then so too does the power output of your panels halve.  But did you know that a partial bit of shading, on only perhaps 10% of your panel, can reduce its output by 50%?  That’s an amazing but observably true issue.  There’s a good discussion about that issue here.

There’s another related factor to keep in the back of your mind as well.  Not all the power your panels will create is necessarily generically usable power.  For example, let’s say you have 2kWh of power produced in a day – that seems like a meaningful chunk of power.  But that doesn’t mean you can run a 1500 watt appliance for over an hour, because perhaps the power is trickling in at only 300W, over a seven hour period.

You’ll never be able to run your 1500 watt appliance from the 2kW of power you got that day, unless you feel the power into a battery bank over the day and then take it all out at the 1500 watt rate – oh yes, and if you do that, you’ll then have to factor in the additional inefficiencies of converting from the AC power to DC power for the battery charger, then the loss in charging up the battery, then the loss in discharging the battery, and the loss in converting the battery DC power back to AC power for your appliance.

Summary

So, your 250 watt panel will probably never ever give you 250 watts of power, under any conditions.  We’d suggest that you use the Californian PTC test results to convert your panels’ claimed power outputs into more realistic output levels, and then reduce those by at least 10% to convert from panel power output in DC to actual AC power available in your home.  In other words, expect less than 200 watts – in best case conditions – from your 250 watt panels, and in worst case conditions (but still with nice sunny weather) you could be dropping down closer to 150 watts as your theoretical maximum.

The bottom line for us as preppers, and remembering we are planning for a future where solar panels aren’t just a fashionably nice ‘green’ supplement to our normal power from the utility company, but rather are our only power source, is this :  Massively over-build your solar array, because no matter how big it is/becomes, it will disappoint and leave you wanting more when you actually start living off the power.

This is a further part of our series on solar energy.  Please also visit our sections on energy in general and solar energy in particular for more related articles.

May 122014
 
Let our spreadsheet save you the need to employ a team of clerks to calculate your solar energy situation.

Let our spreadsheet save you the need to employ a team of clerks to calculate your solar energy situation.

One of the most important things for you to get right is balancing your retreat’s energy requirements with its energy production and storage.

These three variables – how much energy you use, how much you produce, and how much you store – are all dependent on each other, in a complex series of interlinked relationships, and all you really know for sure is that you never want to run out!

But trying to match together the theoretical output of your solar panels, adjusting for changing weather and sunshine during the year, and adding in some energy storage capacity (batteries or whatever) to tide you over the times when your solar power isn’t producing (every night and much of every day during the winter, too) is a confusing and difficult process, and it just becomes harder still when you try to answer such questions as ‘should I add more panels or more batteries’.  Oh yes, and you’ll also want to know the costs of each different way of designing your system too.

In truth, it is hard to avoid getting out a big piece of paper, a pencil or two, some erasers, and a calculator, then devoting hours to running through the specifics of this.  But we can help you to get very close to knowing the answers to these questions without having to spend too much time or hassle doing so.

We’ve programmed up an Excel spreadsheet that allows you to simply enter in the information related to your retreat and your planned power generating, storage, and consumption numbers.  Then it will instantly do literally thousands of calculations and tell you if your scenario will work or not, and show you where the energy shortfalls will be, and allow you to then try ‘what if’ scenarios such as ‘what if I add another panel’ and ‘what if I add another battery’ to get as close as possible to the point where you can confidently predict you’ll never run out of power, no matter how bad the weather.

Of course, once you’ve worked through the spreadsheet, we suggest you then do the ‘real thing’ and calculate the exact specifics for yourself, but at least the spreadsheet will zero you in on the critical parameters and so you only need to confirm the spreadsheet, rather than start from scratch.

We have a link to the spreadsheet for you to download (and we’ve even filled it with some typical values for Coeur d’Alene so you’ve got a reference point to start from) and some pages of explanatory notes to help you know what to fill in and how to interpret the results, all starting from this page here.

This is the first time we’ve created a spreadsheet for you to work forward from.  Do you like it?  Is it helpful?  Would you like more (and, if so, please give us suggestions).

Note – while we are proud of the spreadsheet and consider it very helpful indeed, it is not perfect.  Please be sure to understand its limitations, which we list also on the linked page.  If the spreadsheet proves popular, we’ll improve the sophistication of the model in the future.

Oh yes, if it all gets too much like rocket science, we can help, over and above the large book sized amount of information we’ve already published on the site!  We are happy to consult with you and do most of this on your behalf, and to walk you through the personal preference/lifestyle choices that we can’t make for you.  Rates for our energy consulting services are reasonable and start at $250.  Let us know if we can assist.

May 102014
 
Chances are you'll end up choosing to cover most, if not all your roof with solar panels.

Chances are you’ll end up choosing to cover most, if not all your roof with solar panels.

This is a further part of our series on solar energy.  Please also visit our sections on energy in general and solar energy in particular for more related articles.

What makes a roof better or less suited for having solar panels mounted on it?  How should you design a new retreat structure, and/or, if looking at buying an existing dwelling, how do you know if it is optimized for solar?

Answering these questions is reasonably straightforward.  To start with, if you are looking at buying an existing retreat structure, it absolutely must have a southerly facing roof and an unobstructed view of the southern sky from directly south to about 75° either side of directly south.  You don’t need a full 180° of clearance, but anything much less than 75° either side of south means you’ll start losing some morning or evening sun.

Ideally the roof should have a fairly steep pitch on it.  The ideal angle for solar panels is to have them angled at the same number of degrees as the latitude the panels are at.  That means, if you’re in a northern state, you probably want to have a 45° angle, or even possibly slightly more.  If you think of a line between the two Portlands, the one in Oregon is at 45.5° N and the one in Maine is at 43.7° N, that gives you an intuitive feeling for your likely latitude, and remember that much of the Canadian border follows the 49th parallel, ie, 49° N.  To be more exact, you can instantly see any latitude anywhere from Google Earth and other mapping programs.

It is acceptable to have a somewhat flatter pitch (or a steeper pitch, but that’s less likely!), but once your pitch starts to be more than perhaps 15° away from your latitude, you’re going to start to feel a loss in solar energy production.  A 15° differential will cost you 3.5%, and the loss of power starts to quickly rise from that point forward.

If you are going to build your own retreat, choose a lot that will allow you to build with this southerly aspect, and design your roof for as close to your ideal pitch as is practical.  One thing is likely – you’ll be getting a lot of attic space that way!

Indeed, if you don’t have height restrictions, rather than having a typical ridge line roof with two equal roof sides rising to meet in the middle, why not consider a single sloping roof, going all the way up to the top.  This would give you a lot of extra space above your top level in your structure, and while this space would be facing to the north rather than south, it could surely be used for just about any normal purpose.

How Much Roof Area Do You Need?

Now, the next question becomes either ‘how much roof area do you need’ (if you’re designing a new dwelling) or ‘how much power can you get from the roof you have’ (if you’re buying an existing retreat structure.

The answer to both questions is very much ‘it depends’.  But there are some simple rules of thumb you can use.

At present, it seems that a typical solar panel measures about 39.13″ x 65.04″, and typically generates about 250W according to its official specification sheet.  Some panels will give you fewer watts for this panel size, and some higher priced ones will go up to 275W for the same size.

The panel is close to 18.35 sq ft in size.  So, divide 250W by 18.35 sq ft, and here’s a rule of thumb :  Ideally, with reasonably efficient solar cells in the panels, you can get about 13.5 watts of solar power per square foot of roof area.  If you make adjustments to allow for not every square inch of roof space being usable, and leaving some maintenance walkway space and such like, we’d probably suggest that for quick guesstimate calculations, you figure on 11 watts per square foot of roof.

A 250W panel, which seems to be about the sweet spot for price vs performance, will cost about $250 (plus the associated costs for wiring, installation, control systems, and so on).  This points to another rule of thumb – figure about $1 per watt of panel capacity, plus more to install, etc, the panels and power from them.

Remember that your total roof area will be greater than the footprint of your dwelling.  The slope means it has more length on it, and there is probably some overhang that adds to the roof dimensions too.  But remember also to deduct any parts of the roof that aren’t southerly facing – the ‘other side’ of a typical two-sided roof, the ‘hip’ sides of a hipped roof, and so on.  Also, if there are corners and sides to your roof, possibly the sides might shade the main south-facing roof portion, potentially almost halving the power production on areas that would be shaded.

You probably have a target amount of power generating capacity you would like for your retreat (see our article on ‘How Much Solar Power Generating Capacity Do You Need‘ for more discussion on this).  Multiply your roof square footage by 11, to see an approximate maximum generating capacity for your roof.  Is that above, below, or close to your target capacity?

If your roof clearly has more than enough space for the generating capacity you need/want, then you can relax, and proceed with all the other things to consider when evaluating current retreats or planning your own custom retreat.

If your roof is marginally close to meeting your power requirements, maybe you should calculate things more carefully.  In this case, we suggest the easiest thing to do is to get scissors and paper.  Cut out a large shape that represents the portion of your roof that is southerly facing.  Then cut out, to the same scale, the number of 250W panels you want to place on your roof (maybe, to make things quicker/easier, cut out larger shapes that represent strips of 2, 3, 4 panels).

Lay the panel shapes out on the roof shape.  Leave some aisles for you to walk along (or up and down) so that you can access your roof for maintenance (hopefully seldom) and cleaning (depending on where you live, cleaning will be a reasonably regular activity).  We suggest you allow about 2ft wide corridors, and you design things so you’re readily able to reach panels with a ‘window washing’ type extendable handled cleaning device (which indeed might be a window washer).  Maybe you can plan to reach out 9′ or so from where you are standing.  So that would allow for aisles every 18′ if you access the panels from both sides, and perhaps you’d want the first aisle 9′ from the edge of the roof.

We don’t know why, but we see very few roof installations that leave aisles to make it easy to access the panels, but we feel this to be essential.  It doesn’t take much dust or dirt or leaves or branches or bird poo or whatever to massively reduce a panel’s power output, so we believe regular panel cleaning is essential.  Perhaps the designs with no walk-ways assume that you’ll do the cleaning from a ladder or from the other side of the roof, and those are both possible options.  But if you’re like us, the easier something is to do, the more likely you are to do it, and so we’re keen to make this as easy as possible for us.

So, lay out the panels as best you can and see how many will fit.  The good news is the panels can be laid in either direction – long side horizontal (ie ‘landscape’) or long side vertical (ie ‘portrait’).  While it mightn’t look so nice aesthetically, you can even have a mix of different orientations, any way that will allow for best space utilization.

Measuring Roof Slope and True Roof Surface Area

If you can conveniently climb onto your roof and safely walk around on it, then the easiest thing to do is measure it directly.

But if this is not so practical, you’ll need to measure what you can on the ground and then adjust based on the roof slope for the actual roof surface area.

There are two typical ways of measuring roof slope.  One – less common in the US – is to talk about the angle of the roof slope.  The other is to talk about the slope in terms of units of vertical rise per so many units of horizontal run.

You probably know – or can easily measure – the horizontal length of the building footprint, and you also can probably measure the vertical rise.  It is also possible to measure the degrees of inclination with only some relatively simple tools, but for most of us it will be easier to measure the horizontal length and rise.

Let’s look at a worked example.  Say you have a roof that has a 30 foot ‘footprint’ – ie, it covers 30 ft of horizontal level floor.  It has a single ridge in the middle, and the rise from either end to the middle is 6 feet.

If you remember way back to your trigonometry days, you might remember Pythagoras’ Theorem for finding the length of the third side of a triangle.  The sum of the squares of the other two sides equals the square of the hypotenuse, right?  And in the case of your roof, you now know the two sides around the right angle (ie 6 feet for the rise and 15 ft for the horizontal length).  So

62 + 152 = 36 + 225 = 261, and √261 = 16.2

The roof length is 16.2 ft – not much more than the length on the ground in the case of what would be a fairly moderate slope on the roof.

Oh, and for the sake of completeness, if you do know the angle of the roof and the horizontal length to the ridge point, then you can calculate the roof length by the formula

Roof length = Horizontal length divided by the cosine of the angle.

For example, a roof with a 30 degree pitch and a 15 foot horizontal length to its ridge would have a length of

15/cos(30°) = 15/0.866 = 17.4 ft.

A Sample Calculation

Say you have a 1250 sq ft building footprint (perhaps 25′ x 50′).  Say you extend your roof one foot over this footprint for eaves/overhang (generally it is common to have greater overhang).  And you give the roof a 45° degree pitch.

Of course, you want the long side of the house to be facing south.

If you have a standard single ridged roof, with no hips, and if the roof is in equal halves about the central ridge, then the actual dimensions for each half will be 52′ long (the 50′ width plus an extra foot at each end) and the width will be 37.4′ (the 25 ft flat length becomes a 35.4 ft length on a 45° angle, plus an extra foot of overhang at each end).  But remember that only half of this is facing the sun, so in total, you have 972 sq ft of roof area facing the sun.

Now let’s allow for some maintenance aisles.  Should these lanes run along the roof, or up and down it?  We’re not sure which is better, you can decide.  But let’s simply, for now, set aside 20% of the gross area to leave you room for these aisles.  So your 972 sq ft of panel area has a net usable area of 778 sq ft.

We’d round that down a bit further and call it 750 sq ft.  Or, alternatively, because you are using real dimensions rather than trying to give a generic example, now is a good time to start mixing and matching the actual dimensions of panels to the space on your roof.

For this exercise, we randomly chose a fairly standard size panel, measuring 39.13″ x 65.04″, which we’ll call 40″ x 66″ for our calculation.  These panels are rated at about 250 watts, which means that each ten square inches of panel is giving you almost 1 watt, or, if you prefer, each square foot is giving you about 13.5 watts.

Now let’s first do a ‘perfect world’ calculation.  Our roof has 52′ x 18.7′ dimensions, or 624″ x 224″, which is 139,776 square inches.  Our panels are 2640 sq inches each, so in theory, we can somehow fit up to 52.9 panels on the roof.  If we do the quick rule of thumb and reduce that by 20% (for aisle-ways), that points to 42.4 panels, which we’ll round down to 42.

That suggests our roof can provide a maximum of 250W x 42 panels, = 10.5 kW of power.  That’s actually a reasonably good number for most retreats and most purposes.  These panels would cost about $9,500, plus extra for mounting accessories, mounting, wiring, and so on.

If you were keen to maximize the power from your roof, you could get slightly more efficient panels that generate 275 watts from the same surface area.  But these more efficient panels are also very much more expensive – your cost for 42 panels is likely to increase from about $9,500 up to about $14,600, while your power output will go from 10.5kW up to 11.55 kW.  You’re paying an extra $5,100 for 1 kW of extra generating capacity – that’s a lot of extra money, and maybe it is better to think about spending the money to adapt your roof so it can accept four more of the standard panels (which would add the same additional capacity), or perhaps, use the money to build a shed and put the panels on top of that.  You need an extra 75 sq ft for the four extra panels.

Another approach is to have more of your roof pitching up in the southerly direction, and less or none in the northerly direction.  This will raise the maximum height of the structure, but if that’s not a problem, then go for it.  You’re sure to find a use for the extra internal space you are creating, too.

Personally we generally prefer to have more low efficiency panels rather than fewer high-efficiency panels.  Not only is it cheaper, but the loss of a single panel is not so serious, and our sense is that lower efficiency panels might be more reliable and ‘less stressed’ than higher efficiency panels.  But we have nothing to back up that perception.

If your target power generating capacity is around 10 kW, then you don’t need to do anything more at this stage.  You know that for 10 kW, you’ll need 40 panels, and you know that your roof has enough space for up to 52 panels, depending on layout and service lanes, so clearly that is going to work.

But if you are keen to get every possible watt you can, and you’re thinking of paying a great deal more for higher efficiency panels, now is the time to do an actual layout diagram for how your roof could be set out, using the cut out shapes.

Summary

We provided a couple of rules of thumb in this article.  There’s one more rule of thumb, or perhaps assumption, that seems fair.  It is probable that you’ll want to cover your entire roof with solar panels; especially if you have a multi-level retreat (ie more total floor area, and more living space, but with a smaller footprint and roof area).

The information in this article helps you understand how to calculate how many solar panels you can get on your roof.

May 022014
 
A diagram showing how a fuel cell works.

A diagram showing how a fuel cell works.

This is a further part of our series on solar energy.  Please also visit our sections on energy in general and solar energy in particular for more related articles.

Chances are you’ve not even thought about hydrogen powered fuel cells as part of your retreat energy strategies.  For most purposes, fuel cells would indeed not be a good choice, but there’s one special scenario where they might be of relevance – as part of a way to store energy.

But, before you rush off to your local fuel cell store to buy a dozen (as if such a place exists!), you might want to read through this article and in particular, appreciate that whatever the advantages of fuel cells may be, there’s one also a disadvantage.  The technology for a ’roundtrip’ transfer of energy from electricity to hydrogen and back to electricity is expensive and not very efficient (on the other hand, it is not necessary any worse than other alternatives open to you).

You could be excused for stopping reading at this point, and we write the rest of this article mainly to provide a reference point you can track from.  We are sure that the technologies involved will improve over time, and you can match new products with the information here to see how much closer to practical affordability things are progressing.  Of course, if you do come across a relevant new improvement, please let us know so we can update the information here.

We’ll explain a bit about fuel cells in this article, and look at the plus and minus issues associated with storing your surplus energy (should you be so fortunate as to have any, of course!) in hydrogen and then converting it back to electricity via fuel cells.

What Fuel Cells and Hydrogen Energy Storage Involves

A fuel cell can be thought of as using the opposite process to that which causes the electrolysis of water.  When you electrolyze water, you use electrical energy to separate the water into its component hydrogen and oxygen.  When you use a fuel cell, you combine hydrogen and oxygen to create water and you get electrical energy produced as part of the process.  Other materials can also be combined, but hydrogen and oxygen is the most common, particularly because this means you only need to store the hydrogen, obtaining the oxygen from the air all around us.

Note that you could also run an internal combustion engine on hydrogen fuel, but this is nothing like a fuel cell.  A fuel cell has no moving parts and generates little heat and almost no noise, an internal combustion engine takes chemical potential energy, converts it to heat, and then converts the heat to motion, and then converts the motion to electricity.

Ecologists like fuel cells because the byproduct of running a fuel cell is simply water.  No noxious/toxic nitrogen or sulfur products, and no carbon releases.  What’s not to like about that (or so the ecologists ask/tell themselves).  But, as with most things, there’s a lot more to fuel cell technology than simply ‘burning’ hydrogen and oxygen and getting water and electricity as a result.

Fuel cell technology is both old and new.  It has been around in experimental form for a long time – the first were invented in 1838.  More recently, fuel cells have been used to generate power on the space station, and in experimental fuel-cell or sometimes termed ‘hydrogen powered” vehicles.  Fuel cells, in miniature form, are even used in portable electronics.

Not all fuel cells use hydrogen.  There are other chemical processes that can also be used (if the fuel cell is designed for it of course), but we are focusing on hydrogen fuel cells here because on balance, the ’roundtrip’ to generate hydrogen, store it, then use it in a fuel cell is probably the easiest and best for prepper purposes.

Although based on old technologies, fuel cells are also more in an experimental than a commercial state of being at present.  This is because there are issues with cost and efficiency that currently make them impractical for any type of regular normal use, but while the efficiency levels are very low, the promise of boosting efficiency holds out an exciting hope that fuel cells may become more commercially viable in the future.

Furthermore, if money is not a constraint, then the ability to store hydrogen for extended times with little loss is a huge plus.  Most other energy storage systems are not as practical as hydrogen if you want to store the energy not just for a day or two but for many months.

Hydrogen Related Issues

Even if fuel cells themselves become more efficient, there’s another step in the process that needs a lot of additional optimization – collecting/creating hydrogen and then distributing it to refueling points.

It is important to realize that while hydrogen is the most abundant element on the planet, and oxygen is all around us, that does not mean the cost to power a fuel cell is negligible.  Most of the hydrogen out there is currently ‘locked up’ in water (which is, as you doubtless already know, H2O) and extracting the hydrogen from water (or from natural gas – another rich source – CH4) – is expensive, as can also be the technologies to store and transport hydrogen.

On the other hand, as long as you have hydrogen stored in a leakproof container (slightly harder than it sounds!), it will keep forever.  This is the same as propane, but quite unlike petrol or diesel (see our discussion about storing liquid fuels), and also quite unlike other energy storage methods such as batteries or accumulated reservoirs of water for hydro power.  This is clearly a good thing.

The relevance of hydrogen storage and fuel cells for us preppers is as another way of storing energy.  If you are preparing for only a level 1 or low-grade level 2 event, maybe you’ll cover your energy needs by simply buying a generator and laying in an adequate supply of fuel for it, and possibly stick a couple of solar panels on your roof as well.  That’s a fine way to proceed, and it allows you to reasonably closely match your power needs as they vary during each day and night with the supply of power from the generator.  In such cases, however, you’ll not really ever find yourself in a situation with ‘too much’ power and wanting to find some good use for it.

But most of us, no matter what outcomes we are preparing for, will choose to use primarily renewable energy sources (ie wind and solar) for much/most/all our energy needs.  The huge problem with these energy sources is that their output varies widely, with the weather, and in an unpredictable manner.  As we discuss in articles such as ‘How Much Solar Power Generating Capacity Do You Need‘ and ‘How Much Extra Emergency/Reserve Capacity Does Your Solar Power System Need‘, it is necessary to develop more powerful than necessary generating systems so that they will provide close to sufficient power, even with very little sunlight (or wind).

The happy flipside of this is when the wind is blowing in its sweetspot speed zone, and the sun is shining brightly onto your solar panels in a cloudless sky.  All of a sudden, your power generation is providing you with two, five, maybe even ten or twenty times the power and energy you need.

There’s no such thing as ‘too much power’ of course, and there’s no problem or harm to your system if you simply choose to ignore the extra power that is coming from your generating system and ‘waste’ it.  But maybe you might also look at the downside of sometimes being very short of energy, and seek a way to capture and save/store some of this spare energy to then use when your system is struggling to provide the energy you need.  There are a number of ways you can do this, and the simplest is to go out and buy some more storage batteries.  But maybe you feel the need to shun simple, and prefer to seek out complex solutions!  Or, more seriously, maybe you want to spread your risk by having multiple technologies for every part of your retreat and its mission critical systems, and in such a case, a second method of power storage in addition to batteries might be worthy of some more investigation.

It would be possible to use this extra power to store hydrogen, and then when you need to use the stored energy, run the hydrogen through a fuel cell to regain the electricity.  That’s the simple theory of it, anyway.  Let’s look a bit more now at how it would actually work, and what problems you could run into.

Let’s think about this in three steps.  The first is how to ‘get’ hydrogen.  The second is how to store it.  And the third is using the fuel cell(s) to convert it back to electricity.

1.  Getting Hydrogen

There are two possible and somewhat practical ways of getting hydrogen.  The first involves the electrolysis of water, the second involves steam reforming of natural gas (ie methane); this latter method works well for large plants, but not so well for smaller domestic production.

So, if you look at electrolysis, while you could build your own electrolysis plant, perhaps you’ll simply choose a turnkey unit.  Ebay sells hydrogen generators, and we saw a listing for one that requires a maximum of 400W of power to generate up to 5L of hydrogen a minute.  In other words, if we believe its specifications, 1kWh of power would generate 750L of hydrogen.  Ebay has this unit listed for $12,000, and it is being sold by a Chinese supplier.

But, be warned.  Anything that talks about ‘up to’ has immediately given itself an enormous ‘loophole’ to invalidate its claim.  Up to 5L a minute includes all numbers less than that, doesn’t it.  We just do not believe, for an instant, this unit will consistently create 5L of hydrogen while consuming electricity at a rate of only 400W – if it was to do this, it would be unbelievably efficient, and several times more efficient than leading branded products with more credible specifications.  You’ll doubtless be shocked and horrified to learn that you need to tread cautiously when trusting the word of Chinese suppliers selling products through eBay!

Before we move on, let’s just point out one other thing about this unit.  It only draws up to 400W of power.  You are likely to have several kW of spare power capacity at good times that you want to divert and store.  So you might conceivably spend $36,000 for three of the units, and that still only gives you a way to divert 1.2 kW of power.  If you wanted to be able to divert 5 kW, then you’re looking at $144,000 for enough of these units, and clearly that’s no longer a sensible approach.

Let’s now look at another product with more believable specifications.  A Hydrogenics HyLyzer unit.  These will product either 1 or 2 Nm3/hr of hydrogen – that’s basically a fancy way of saying 1000 or 2000 liters.

The key part of the specifications given here is the disclosure that the unit consumes electricity at a rate of 6.7 kWh per 1000 liters of hydrogen created.  So a single unit would soak up your spare power at a rate of 6.7 kW.  To put that another way, each kWh of electricity creates about 150 liters of hydrogen.  Units can usually be run at maximum production rate (to use up the maximum spare power) or at lower rates (if you have less spare power).

Heavier duty larger capacity units (of a size probably impractical for us to consider) can be somewhat more efficient, perhaps requiring only 5 kWh per 1000 liters of hydrogen.

We understand but have not been able to get confirmation from Hydrogenics that these units cost in the order of $40,000.  Here are some apparently not quite so good units and their prices.

2.  Storing Hydrogen

The good news is that hydrogen is very light.  Indeed, the weight of the hydrogen you store will be only a very small fraction of the weight of the tanks you have to store it in.

You can choose from various different sizes and strengths of gas tanks.  They can weigh as much as 300 lbs each (for a typical ultra high-pressure steel ‘6K’ tank with a 42.48L storage capacity) and can store hydrogen at pressures of up to 10,000 psi (700 Bar).  At 700 Bar, you are storing just under 38 gm of Hydrogen per liter of tank space, at 200 Bar, you are storing about 14 gm per liter (and about 7 gm/L at 100 Bar).  In addition to heavy bulky steel ‘bottles’ (sometimes with side walls an inch thick) there are more modern containers with high tensile carbon fibres wrapped around the inner bottle, allowing for smaller lighter containers with the same strength as steel.

We prefer medium rather than ultra high pressures – less energy is required to compress the hydrogen to store it and there is less risk of tanks exploding and less stress on adapters and intermediate pressure regulators.  On the other hand, the less the compression, the greater the number of tanks you need.

A possible compromise would be tanks weighing about 150lb each and holding about 7250 liters of hydrogen.  Each of these tanks has enough hydrogen for about 9 kWh of electricity, and costs in the order of $400.  Depending on your perspective, that’s either a lot, or very little.  To put it in perspective, a 12 kWh storage battery, that would be suitable for about 9 kWh of actual power, costs $3000.  So the cost of tanks to store hydrogen is massively less than the cost of batteries, and furthermore, whereas batteries have a finite life and need ongoing trickle charging to maintain their charge, once you’ve filled your gas bottle full of hydrogen, you can pretty much forget about it for a long time into the future.  And you could empty and refill it very many times before it became unreliable (even though we don’t anticipate you doing this much more than once or twice a year).

A suitable compressor is likely to cost in the order of $5000.  Add another thousand or two for miscellaneous piping, regulators, and other incidentals.

In addition to storing compressed hydrogen, it is also possible to store it mixed in with other chemicals (metallic hydrides), or in liquid form.  The former method is complicated but allows for dense storage of hydrogen, the latter method is massively more ‘costly’ in terms of energy needed to liquify and cool it.  We like the simplicity of just compressing the gas.  Less to go wrong, which has to be a major consideration when planning for any sort of emergency backup scenario.

Is Hydrogen Dangerous?

Talking about things going wrong, some people are unnecessarily alarmed at the thought of storing hydrogen.  They have this mental image of the airship Hindenburg in flames, and that colors their perception of hydrogen as a safe fuel.

In actual fact, the most spectacular part of the Hindenburg’s fire and demise was not the hydrogen burning, at all.  It was the very reactive and flammable outside skin of the airship, and also the diesel fuel stored on board.  Yes, the hydrogen did burn, but quickly and relatively harmlessly compared to these other two fire sources.  The Hindenburg would have burned almost identically even if it was full of ‘safe’ helium (the fire is believed to have been started by lightning igniting its outside skin).

Hydrogen is actually less flammable than regular petrol.  Gasoline will burn when it reaches a temperature of 536°F, hydrogen requires a higher temperature of 932°F.

The best thing about hydrogen is that it is very much lighter than air (about 15 times lighter).  If any hydrogen ‘spills’ or leaks or otherwise escapes, it simply shoots upwards, like a cork held at the bottom of a tub of water and then released.  As long as the release of hydrogen is either outdoors or in a structure with venting that allows the lighter than air hydrogen to escape to the outside atmosphere, there will be no problem.

3.  Fuel Cells

Fuel cells have a lot going for them.

They are easily twice as efficient than internal combustion engines, because they directly ‘convert’ hydrogen to electricity, whereas an internal combustion engine converts hydrogen (or any other fuel) first to heat and then to mechanical energy and then from that to electrical energy (that’s three conversions instead of one, each with issues and inefficiencies).

They are quiet, compact, and have no moving parts, making them potentially very reliable.  Indeed, these fuel cells talk about needing maintenance only once every five years.  On the other hand, there is a wide variation in cells as between ones designed for occasional/intermittent/light use, and those designed for heavy-duty ongoing use, and we sense that many fuel cells are still only one or two steps away from experimental and possibly not yet ready for a long hard life ‘in the field’.

It seems that fuel cells commonly consume about 800 liters of hydrogen per kWh of power generated.  Some can go down to the low/mid 700s, and we’ve seen others climb up over 1000 L/kWh.  Compare that to creating 150 liters of hydrogen per kWh of power consumed, and you’ll see that you are getting little more than one sixth of the power back that you used in the first place.

Furthermore, you need to allow for minor other power losses in the total process from solar cell to hydrogen to fuel cell to a/c power in your retreat.  For example, these efficiency ratings are the best possible, assuming the units are running in their ‘sweet spot’ – if you reduce their loads, then their efficiency may drop off.

No allowance is included for the energy cost to compress the hydrogen, for any leaks and losses during transporting the hydrogen from electrolyzer to fuel cell and storing it, nor is any allowance included for other things like energy for cooling systems, and other inputs such as electrolyte and coolant.  We’re also not allowing for more energy loss when the DC power out of the fuel cell is converted to AC power.

Here are some fuel cell systems and their prices.  Here’s another one, slightly less efficient.

Hydrogen Storage – Costs, Costs, and Benefits

So, to summarize, it seems that a hydrogen storage system will cost you in excess of $40,000 for a hydrolizer, another $20,000 for a fuel cell, and let’s say $40,000 for compressors, storage tanks, and everything else.  In other words, you have about a $100,000 cost to get a reasonably sized system established.

This is expensive, and it is also inefficient – you get back about 1 kWh for every five or six you use in the first place, and you’re using a system that can only use power at a rate of about 6.7kW; you’d need to add another $40,000 to your installation cost to be able to recover power at twice the rate.  On the other hand, does efficiency really matter if the energy is otherwise being wasted?

The 6.7kW rate is interesting, it coincidentally means that for every hour you are capturing energy at this rate, you get a net of about 1 kWh of energy for reuse in the future.  Let’s say that in the summer, you have four hours of ‘bonus’ power each day for 150 days, that means you’ve managed to harvest a usable 600 kWh of energy to use during the winter.

That’s actually not as insignificant or disappointing an outcome as you might at first think.

You would need almost 500,000 liters of hydrogen stored in this example so as to get the 600 kWh of output (or, if you prefer, 40 kg).  On the basis of tanks weighing about 150lb each and holding about 7250 liters of hydrogen, this would be a tank farm of about 70 tanks.  Compare that to a comparable battery setup with a similar number of batteries – there’s probably not a huge difference in size (and, in case it matters, the batteries are heavier).  The big difference is in total system cost.  Whereas the hydrogen system has a high cost to get started, it has a low variable cost as you increase its storage.  The battery system has a low-cost to get started, and a high variable cost for each extra battery.  You’re looking at the better part of a $250,000 system to store the same amount of power in batteries as you could with a $100,000 hydrogen system.

So, yes, a $100,000 investment would get you a setup that could store 600 kWh of energy.  Add another $40,000 for more hydrogen generation capability, and more for additional storage as your budget would stretch – perhaps another $80,000 to have 1.2 MWh of energy that is stored over the summer and available over the winter ($180,000 for hydrogen compared to perhaps $450,000 for batteries).  Now the benefits of a hydrogen system start to become very apparent.

There’s another point in favor of hydrogen storage too.  If you were to have an enormous 600 kWh battery storage resource, you might find it ‘costing’ you as much as 6 kWh a day, every day, in trickle charging, to keep the batteries maintained and in good condition.  That’s not a problem in summer, hopefully, but in winter, that is power you can ill afford.  To put that in perspective, you want your 600 kWh of power to last you for 150 days of winter, but during those 150 days, you’ll need 900 kWh of power just to maintain the batteries.  That doesn’t work, does it!

Batteries are the best solution for storing relatively moderate amounts of energy for relatively short periods of time – a day or two up to a week or two.  But for large amounts of energy, and longer periods of time, they start to become less appropriate, as we just saw in the above example.

One further comment.  If we were to invest in a hydrogen system, we would definitely want to make sure we had redundancy – both plenty of spares and ideally two hydrolyzers and two fuel cells.  So either buy two smaller units, or increase your investment still further by having additional larger units as part of the system.

UPDATE March 2018 :  Costs for battery storage have dropped, and probably now using a Li-ion bank of batteries would allow for lower trickle charging needs too.  Without redoing the math, we do note that all major commercial installations for storing surplus energy seem to be moving to battery solutions rather than hydrogen solutions.

Battery technology has improved substantially in the four years since this was written, and promises to continue improving, with the potential for major stunning breakthroughs at any time.  On the other hand, hydrogen and fuel cell technology is more mature and is limited by inviolable physical and chemical constraints that mean there is no perceived potential for similar improvements in hydrogen technology.  We suspect that hydrogen as a way of storing/banking electricity is a technology that has come and gone, never to return.

Summary

If you want to store less than, say, 100 kWh of energy, and for a relatively short amount of time (say, less than a month), then probably batteries are your best choice.  But for large amounts of energy storage – indeed, for almost unlimited amounts, and for long-term storage, a hydrogen based storage system comes into its own.

If you’re seriously considering such a system, check the current pricing and the best models out there.  Our sense is that values and capacities are slowly evolving and improving, making the appeal of these systems become greater and greater over time.

Apr 292014
 
This backup battery setup looks good, but there's one huge mistake the prepper made in developing it.  Do you know what it was?  The answer is at the bottom of the article.

This backup battery setup looks good, but there’s one huge mistake the prepper made in developing it. Do you know what it was? The answer is at the bottom of the article.

This is a further part of our series on solar energy.  Please also visit our sections on energy in general and solar energy in particular for more related articles.

There are two ways of developing a solar system for a retreat.  By far the simpler is to simply go out and buy the biggest bestest system you can afford, and whatever you end up getting is what you have, and you’ll adjust and survive as best you can with what you have and the energy it provides.  There’s little danger you’ll end up with ‘too much’ solar power!

The more complicated way is to work out how much power you need to live your life to a certain standard of comfort and convenience, and to buy sufficient power generating capacity to provide this.

If you’re just going to buy a system and hope for the best, we’d urge you to buy way more capacity than you think you might need, and way more than the sales people are telling you to get.  That’s an interesting reversal – usually sales people tend to oversell and you can buy less than they tell you is necessary, but in the case of solar power, the chances are the opposite may be true (we guess salesmen undersell because the true full costs and complications can become rather daunting).

Even if you’re just going to buy a system and hope for the best, you should still read through the rest of this and the associated articles in our solar power series, so you have at least some idea of the big gap between claimed power generating capabilities and the actual energy you are likely to receive (sometimes there can be a five-fold discrepancy), and can create some sort of realistic expectation for what you’ll actually be able to do with your system.

In our article ‘How Much Solar Generating Capacity Do You Need‘ we looked at some of the considerations needed to match your power and energy requirements (two similar but different things, as explained here) with what to expect from solar panels.  The big problem is that the output from solar panels varies more or less proportionately with the intensity of the sunlight shining on them, and the total sunlight each day can vary widely, depending on just on season, but on each day’s specific weather.

So although you can work out your average daily needs for power (probably with some seasonal variations) and similarly, you can work out your average daily power generation (again with seasonal variations) the problem is that your actual daily power generation will sometimes be much higher than average (well, that’s not a problem, is it) and on other days, will be much less than average (and there’s your problem!).

The other big issue with solar power is that at night, solar power reduces down to zero.  You need to supplement solar power either with another power source that does work reliably at night, or with a means of storing/saving energy during the day that you can then tap into at night – the most common example of which being batteries.

Clearly, you need to select a system with as much power as possible to keep you going on very low sunlight days.  But there is also a point at which you can switch strategies – by increasing your battery storage capacity, you can provide yourself with additional energy reserve by storing up surplus energy on sunny days, and using that stored energy not only at night, but on days with less than anticipated sun, too.

Within reason, and ignoring some minor issues, the bigger the battery reserve you have, the smaller the actual solar generating system you need, because the batteries are smoothing out the peaks and the troughs as between your energy production and your energy consumption.

Batteries for Bad Days

So, we know that the solar power you’ll generate will vary from day-to-day, and some days will have less power generated than you want, and perhaps even less than you need.

There are three ways to respond to this.  The first is to upgrade your solar generating capacity still further, so that you still get the energy you need, even with much less sunlight.  The second is to have some alternate backup sources of power generation.  The third is to have more battery capacity, so you can smooth out the peaks and the flows of daily power generation with stored battery power.

In this third case, you are using batteries for two purposes.  The first is to take power generated during the day and use it the immediately following night, with the expectation that come shortly after sunrise the next day, you’ll have used up your battery power (at least to the discharge level you set yourself) but that at the same time this happens, energy will be flowing back into your retreat from the solar panels.

The second purpose is to allow for cases when, after discharging your overnight batteries, the sun does not come up and shine as brightly as hoped for the next morning and you need to continue relying on your batteries for some/much of your energy.  (A similar situation is where you generate enough energy during the day for your day’s needs, but not enough for the next night’s needs too.)

The chances are that you’ll have the same batteries doing both tasks – this is not like, for example, a motor home which might have both a starter battery for starting the engine and a separate ‘house’ battery for running appliances from.  At your retreat, you simply increase the size of your battery installation and use them for both overnight and top-up during the day purposes as needed.

This is a good thing for another reason too, because it means on a typical day, you’ll be discharging your total battery system less than you would with a lower capacity system, meaning you’ll have more charge/discharge cycles in total before the batteries eventually die.

How Much Extra Battery Capacity to Add?

To use a similar scenario to that in previous articles, if you need 20 kWh of energy a day, and if you have a system that can generate 25 kW per hour of full sunlight, let’s also assume that on average you expect to get 2 – 2.5 hours of full sunlight a day, and you have configured things on the expectation of it all working right even with only 1.125 hours of full sunlight.

Maybe you could say ‘Well, we know that, no matter what, the sun will always rise, every morning, so even on the worst of all days, there will still be perhaps 15% of the full energy available for harvesting through the system.  That would mean a really bad day would give you 0.34 hours of full sunlight equivalent, and you could get 6.75 kWh of energy from that weak amount of sunlight.

You need 20 kWh of energy a day, so you have a 13.25 kWh shortfall.  You already have a battery resource designed to give you at least 5 kWh of energy for nighttime, so it seems you need to increase your battery resource by another 8.25 kWh of net capacity to give you the ability to work through a really bad day.

Now, what say you have two bad days in a row?  Do you need to add still more battery capacity?  Maybe, you do.  But perhaps not quite as much as you might think.  We’ll assume that, on the second day with very little sunlight, you’ll stop using any electrically powered things that aren’t essential.  You’ll stop using your hair dryer, your dishwasher, and so on.  And on the third day, you’ll cut back even further, so that your 20 kWh consumption figure drops way down to start with, approaching closer to the perhaps 6.75 kWh of energy created.

Don’t get us wrong.  More batteries and more panels are always good, but there comes a point where you truly do have enough, and should think of better ways to spend your prepping budget.

There’s another thought that you need to keep in mind, too.  Your system has been sort of designed to have enough capacity for a day’s normal use, and to recharge your 5 kWh of overnight battery capacity, but if you need it to also be able to recharge another 8.25 kWh of battery capacity, do you need to increase your array size, too?

Maybe yes, but maybe also no.  Here’s the surprising thing.  Sooner or later, the pendulum will swing and instead of having bad days with less than average sunlight, you’ll have a normal day with normal sunlight, and even sometimes a good day with more than normal sunlight.  Remember that, on average, you will get 2 – 2.5 hrs of sunlight equivalent, and you’ve designed your system to work with only 1.125 hrs of sunlight per day, and to withstand any two-day period with only 1.465 hrs of sunlight over the two days.

Maybe the third day will see you back to normal, with between 2 – 2.5 hours of sunlight.  Your 25 kW system will be gushing out so much energy you’ll have your batteries topped up in double-quick time, and you’ll be able to use all your appliances without any care or concern at all.  And the next day, maybe you’ll get 3 or 4 hours of sunlight – you could potentially generate 100 kWh of energy in a case where you only need 20 kWh for your daily needs.

On the other hand, if you now have a much larger battery reserve, they will be consuming a measurable amount of energy each day to keep ‘trickle charged’, and conditioned.  We’d recommend you allow at least 1% of the total battery capacity for daily losses, and depending on the type of system you have, maybe this could rise to 2% or more.  2% of a 13.25 kWh system is 0.27 kWh for battery maintenance every day.

So, where do you draw the line?  For many people, roof size is one practical consideration.  You’ll likely max out your roof space with these types of configurations.  And the other is budget based.  For those of us with finite budgets (and that is most of us) we always need to be juggling our funds between all the different ways we need to spend them, and ensuring that, just like a chain is only as strong as its weakest link, our retreat has no weak links as a consequence of also having some ridiculously strong ones.

Redundancy

There’s another thing to consider in working up the specifications for your solar power system.  You need to have redundancy built-in to the system, so that everything still works as you need it to if any one thing fails.

So, for example, maybe you end up with a net battery power requirement of 5 kWh, as per the example we’ve been working through, and you find a single Solar-One battery which itself has a net 6 kWh of power.  Don’t just buy it and consider your battery needs fully solved, because you have also created a part of your system with no backup/redundancy.

Instead, you should buy two of these batteries.  Or perhaps you find a different brand of battery that has a net capacity, per battery, of 3 kWh.  Don’t just buy two.  Buy three, so you can have one fail and still sufficient battery capacity remaining.

Always buy (at least) one more than you need for anything and everything.  And if it is an item which you have/need many of, consider buying more than one additional.  For example, maybe you end up with 50 solar panels.  Even though the panels are very reliable and low maintenance, we’d be tempted to buy not just one but two or three spares for ‘just in case’ scenarios in the future.

So, buy the system you need, then buy an extra at least one of every item in the system.  Sometimes, it might make sense to immediately deploy the extra items so as to make your system bigger and better right from day one, other times (maybe power controllers, extra cables and connectors, etc), there is no need to do this and you can keep them in reserve, so that they’re not wearing out or getting ‘used up’ or whatever other issues might apply.

In the case of batteries, we’d probably immediately connect up all the batteries we had.  The slight downside is a greater energy consumption every day to keep them charged, but the upside is that you are using a smaller amount of each battery’s charge each night, and so they will last more cycles in total.

Should You Buy More Batteries, or More Solar Panels?

So at what point does it make sense to spend money on more batteries rather than on more solar panels (or vice versa)?  How much of each should you have?

There are several ways to answer that question, and several issues to consider.  Two important considerations are the relative cost of solar panels compared to batteries, and the confidence you have in how predictable each day’s energy production will be.  Another issue may be any constraints you might have in terms of how large a solar array you can deploy – although in theory there is no limit, because assuming you have a reasonable sized property, you can simply add more rows of panels just above the ground if you’ve run out of roof.  In practice, the higher up your panels, the better protected they are from accidental damage of all kinds, and once you’ve filled all your roof area,

Prices for batteries (and the associated control and conversion equipment to charge them, monitor them, and subsequently then change their power to 110V AC for use around your retreat) and for solar panels (and all the various similar associated equipment for them) varies to a certain degree, and it is hard for us to be too definitive about that part of the equation in an article that we hope will remain useful for an extended time.  But to give a quick example of some pricing at present, you could plan to spend about $5000 for a battery and related equipment that stores 12 kWh of energy (for example, these batteries), and/or you could spend about $1/watt of solar panel generating power plus, say, $1500 or so for an inverter, meaning the same $5000 would buy you about 3500 watts of power generating capacity.  Note that you would only want to discharge the 12 kWh of battery to perhaps 75%, so it actually gives you 9 kWh of usable energy.

If we say that on a worst case day, you’ll have under one hour of full sunlight energy equivalent, then your $5000 would buy you either 9 kWh of stored and usable energy in additional batteries, or less than 3.5 kWh of additional energy generated.  All other things being equal, it would seem, with these respective prices, you would be better to spend the money on batteries.

But – and here’s the thing.  All things are not always equal.  If you increase the size/capacity of your bank of storage batteries, you also need to have sufficient generating power to be able to charge them up – and, don’t also forget, that once you’ve charged them up, you then need to give them ongoing trickle charging to keep them charged and in good condition.

Perhaps a better equivalent would be to say that if you buy the 9 kWh of extra usable battery for $5000, you also spend another $5000 to get the 3.5kW of extra solar cells so as to have additional capacity to charge the additional batteries, making a total cost to service the 9 kWh more like $10,000.

So, to compare how you’d spend $10,000, one way you’d get 12 kWh (with 9 kWh usable) of batteries and maybe 3.5kW of solar cells, the other way, you’d get maybe 7 kW of solar cells and no extra battery.  But, remember, you’re planning for a worst case scenario with less than one effective sunlight hour, so the 7 kWh of extra solar cells might only bring you a net 5 kWh of energy, much less than you’ll hopefully have stored and usable in your 12 kWh of extra battery.  It still makes sense, after adding this extra detail, to buy batteries (and some additional supporting solar panels) than just to buy panels by themselves.

Note that these numbers will vary depending on what you already have and what else extra you are looking at getting, and you should do your own sums your own way, but at least this worked example, as of late April 2014, clearly shows that for energy storage strategies, you should go big on batteries.

And now, for an opposite thought.  Rather than looking only at the implications of adding extra batteries and panels, think also of what you already have.  In our example, with 5 kWh of battery storage, and then you add an extra 8.25 kWh for bad days, and then you add a further 12 kWh for really bad days, you also already have 25 kW of panels and a daily/daytime need for only 15 kWh of energy (plus 5 kWh at night).

As soon as you transition from bad weather to average weather, you then have (in this example) 2 hours of full sunlight energy, and the panels will then give you 50 kWh of energy to use and store.  That’s enough for your 15 kWh of daytime use, and to charge your full bank of batteries too, plus more besides.  Maybe you don’t need additional panels – in this situation – to support your extra batteries.

Wow.  So – more batteries?  More panels?  Truly, it totally depends on your present situation and the types of assumptions you are comfortable living with.  Is this the point where we also mention our consulting services and how we can help you ‘tune’ the correct balance of power generating and energy storing capacities?  Rates are reasonable and start at $250.  Let us know if we can help.

Summary

How much reserve power generating and energy storage capacity should you have at your retreat?  Well, that depends on how much you have of both to start with, and then from there it depends on how pessimistic you want to be in terms of semi-random/worst-case scenarios for a shortfall between average days of sunlight and really bad days of sunlight.

We suggest that your ‘normal’ system specification should already embody some conservative projections about how much power you’ll get on bad days in winter, and then from that point, add some extra battery and perhaps some extra panels.  We often see solar panels sold with the assumption they’ll generate 5 hours worth of full-on energy a day.  We tend to base our projections not on these best case summer scenarios, but rather on worst-case winter scenarios, where (particularly in the northern states) you’ll sometimes struggle to get even one hour of full-on energy from your panels each day.

The good news is that even on the darkest dimmest gloomiest day, there will still be a very little bit of energy generated, so even if you exhaust all your batteries, you should still get a tiny trickle of electricity – enough to at least keep some lights going and maybe one or two essential appliances too.

Oh – the big mistake present in the picture of the battery array at the top of the article?  The problem is the batteries are outdoors.  In winter time, the battery temperatures will massively drop, and the colder the battery, the less capacity it can store and return back to you.  You want to keep your batteries as warm as you are – temperatures in the 70s are ideal.

Apr 222014
 
Chances are you'll end up choosing to cover most, if not all your roof with solar panels.

Chances are you’ll end up choosing to cover most, if not all your roof with solar panels.

This is a further part of our series on solar energy.  Please also visit our sections on energy in general and solar energy in particular for more related articles.

We’ve already written an article pointing out something you hopefully already know – solar power is a wonderful energy source for preppers.  If you don’t already have some solar power on tap at your retreat, you should urgently add some.

But – how much solar power do you need?  That’s clearly an essential question to ask and answer.  But answering it is not as easy as asking it (hence this article!).

There are two drawbacks (or, if you prefer, ‘complications’) when it comes to assessing how solar power can meet your energy needs.  The first is that every night, when the sun goes down, your solar power generation drops down to zero, and doesn’t start again until the next morning’s dawn.  You’ll have to create enough energy during the hours of strong sunlight to have sufficient to store (ie in batteries) to see you through the nighttime, too.  This is actually the less serious of the two drawbacks, because this is something you can readily understand and plan for.

The possibly more important drawback is that the power you get from your solar panels depends on how much sun there is, each day.  A bright sunny day without a cloud in the sky means you’ll get lots of power from your panels.  A light haze and some high-up cirrus clouds, while hardly detracting from your overall perception of a bright sunny day, might cut the power you generate by 20%, possibly even 30%.  As for when there are clouds covering the sun, well, your power generated might drop down to barely one-third of what it could in optimum conditions.  It is still clear ‘bright’ daylight and you can see perfectly well, but the clouds have blocked much/most of the sun’s energy from reaching you.

This means when you plan for a solar sourced power supply, you need to have an array that is many times more powerful than you might think you need (so as to provide sufficient power on cloudy as well as bright days), and you need to support the array with a large reservoir of power storage (ie almost certainly batteries – see our article on using batteries to store electricity and the other articles in that four part series).

As an aside, while this seems to be an unfortunate situation, with the total energy ‘harvested’ from the sun varying widely and not very predictably from day-to-day, at least on the very gloomiest of days, you are still getting a small trickle of energy – it is only at night-time that the energy flow stops completely, so you always have some tiny emergency supply.  This is not the case with wind power, where on some days (too windy, or not windy enough) you will get no energy at all.

The good news is that solar panel systems are reasonably affordable, and the further good news is that there is very helpful data to help you understand just how much energy you can realistically get from your panels, if not hour by hour, at least on an averaged basis, daily/weekly (note – this helpful information is usually not the materials provided by the seller of the panels!).

What a Panel’s Rated Power Output Means

The most important part of the specifications of solar panels, for this purpose, is to see how many watts of power it is rated to generate.  Sure, there’ll be other data ranging from size and weight to how it should be mounted, and you’ll also see information on voltages and current ratings, but focus primarily, when planning your array, on the watt rating.

Almost certainly, this watt rating will assume that the panel is directly facing the sun and that the sun is operating at ‘full rated power’ – ie, 1 kW of radiated energy landing on every square meter of panel surface.  As of the current time, it is normal to expect to see panels that are about 15% efficient – ie, they will create about 150 W of electrical power for every square meter of panel surface, in these ideal conditions.

But, as you well know, life is seldom ideal, and the sun doesn’t always shine.  Even when the sun is shining and there are no clouds blocking it, the panel isn’t always directly aimed at the sun (because the sun moves around the sky each day), and the more ‘off target’ the panel is, the less efficiently it converts sunlight to electricity.

Furthermore, it is common for panels to slightly under-perform their rated power output, albeit only by a few percent, and as they age, their power output slowly drops further (by about 0.5% – 1% a year).  And then, from the power output at the panel connector, you start losing power through things such as wire resistance as the power travels from the panel to your appliances, and conversion losses as you convert the panel’s low voltage DC into high voltage AC for your appliances.

If you are storing the panel’s power into batteries, you are again losing power in the process of converting the electricity from the panel voltage to the appropriate battery charging voltage, and then from that into chemical potential energy in the battery (and then back to electrical energy when you start running things from the stored power in the battery).

There are other output modifiers to consider as well.  For example, a partial shadow falling on less than 10% of a panel doesn’t reduce the panel’s power output by 10%, but rather maybe by 50%.

So be very wary of using the rated panel power outputs without understanding exactly what this means and making the appropriate adjustments – ie, probably massively reducing down the number!

One more word of caution.  A panel should only be rated in terms of watts (or maybe kW) of power output.  Sometimes you’ll see this rating then converted into (kilo)watt hours of energy generated per day/week/month/year.  This is a terribly fictitious number, which you should ignore totally, because it is almost certainly based on a ‘best case’ number of hours of bright sunlight per day/week/month/whatever, and as preppers, we never want to build our future on ‘best case’ assumptions, do we!  (By the way, if you’re a bit confused as to the difference between power and energy, please visit our page that explains this – understanding the difference between power and energy).

You must – you absolutely must – create your own calculation for how many kWh of energy you’ll get from your panels, using worst case rather than best case scenarios, in winter rather than summer, and based on the sunlight values applicable to your specific location.  This article and others in the series helps you do so.

Step One – Your Retreat’s Energy Needs

The first thing you need to do is create – well, for want of a better term, an energy budget for your retreat.  How much energy are you going to allow yourself every day or month, and how much of that energy will be in the form of electricity?  The chances are you’ll have a multi-energy strategy for your retreat – you will most likely have some type of wood burning energy source; and for finite term level two situations, you might have propane to power your cooking appliances, you might use solar/thermal energy to heat your water, and so on.

You might also have multiple electricity sources – a generator (suitable for level 1/2 situations, but less so for level 3 because you’ll run out of fuel) as well as solar power.  Perhaps there’s even some wind power, and in very rare situations, you might have a micro-hydro generator too.

For the purpose of this article, our primary focus is on electrical energy needs, and we’re also assuming that this electricity will be sourced solely from solar panels.  Feel free to adjust the process if you’re incorporating other electricity sources too.

So, how much electrical energy (from solar panels) does your retreat need; and perhaps also, how much more than the bare minimum would be nice to also have?

We suggest you work through this question for three scenarios – summer, fall/spring, and winter.  There will be changes in your heating, cooling, and lighting needs for these different times of year.

We also suggest you might wish to assign each item a priority – ‘Must Have’ for the absolute essential items you can’t live without, ‘Should Have’ for some extra items that make life more pleasant, and ‘Would Like but Don’t Need’ for the luxuries that you’d like to be able to treat yourself to but probably can’t.

Go through your retreat, room by room, and inventory every electrical thing in every room, and work out the energy each item requires on a daily basis.  At the same time, as part of an ‘energy audit’ make sure that every electricity consuming item is optimized – for example, do you have LED lighting everywhere, and is it sufficiently but not extravagantly bright?  Do you really need a waste disposal unit in your sink, in a grid-down scenario?  (Answer – almost certainly not, food scraps can be used in a compost heap.)

Sometimes you might have an essential item that you record a certain amount of usage for in the must have category, and additional usage in the less essential categories, too – maybe you plan for an hour a day of radio monitoring as a must have item, but then add another few hours of radio listening in the ‘Should Have’ category, and a few more hours in the ‘Would like but don’t need’ category.

Add it all up, redo for the different seasons, and you’ll end up with your total energy requirements.  If you did this thoroughly, you’ll have nine different numbers (three seasons multiplied by three scenarios).  These numbers are all interesting, but the most important number – the one you’ll start from – is the highest of the three ‘Must Have’ numbers.  This is your absolute bare minimum energy requirement; depending on your budget and other constraints, you’ll of course try to build your system bigger than this.

Here’s a helpful list showing some of the wattages of various home appliances.

Step Two – Your Retreat’s Power Needs

The second thing you need to know is how much power your retreat needs to get the energy you’ve calculated in the preceding section.  You’ll understand this distinction from our article explaining the difference between power and energy.

What is the maximum amount of power your retreat will require at any given moment?  Add together the power requirements of everything that might be on simultaneously, and include an allowance for startup power peaks.

You need to do this only once, for a scenario where you have whatever you want simultaneously operating.  Due to the difference between power and energy, there is no need to redo this for each of the nine scenarios you were studying above.

Either/Or Circuits to Manage Peak Power Needs

As you’ve probably now calculated, although your house/retreat might have modest total energy consumption, it still might have occasional massive peaks in maximum power requirements.  You want to keep your peak power requirement as low as possible, as well as your total energy consumption – this will allow for smaller more economical systems for your property.

One easy way to do this is to have ‘either/or’ circuits in your house.  For example, maybe you have things set up so either your washing machine is on or your dryer is on, but never both.  The easiest way of doing this would be to have the two units sharing the one power outlet, so that only one can be plugged in at a time.

Maybe you also have an either/or setup for your kitchen – either your dishwasher is on, or your oven and stove top are on, but never both.  Perhaps this is controlled from an electrical distribution panel, where you have a switch that can be set to either enable one circuit or the other, but not both simultaneously.

Maybe you also have a switch that powers on either your laundry or your kitchen, so you now have two levels of either/or – ‘Either I have something in the kitchen on or something in the laundry on, and if it is something in the laundry, it is either my washer or my dryer’.  Another example would be either your hot water heater or your furnace – because they both cycle on and off, you’ll not notice the difference at all if you have it set up so they alternate drawing power (as long as their duty cycles are less than 50% each, which they should normally be).  Either the lights in the living areas or in the bedrooms and bathrooms.  And so on, any which way that makes sense for your lifestyle and living situation.

Another and more ‘automatic’ solution would be, if you have, for example, a separate fridge and freezer, would be to put them on timers.  Maybe you have the fridge on a timer that runs for the first and third quarters of each hour, and the freezer to run on the second and fourth quarters.  Better would be to have the fridge running for 13 minutes, then a two minute gap in case the timers get out of synch, then the freezer for 13 minutes, and so on.  Or get a timer that has multiple outlets so it is always in synch with itself, alternately switching on and off different circuits and appliances.

Some other examples of ‘either/or’ scheduling might just require some personal discipline, or a different way of controlling things – for example, don’t do the vacuuming at the same time you do the ironing (both require over 1kW of power).  Maybe you keep the iron and the vacuum cleaner in the same place with a big placard saying ‘Do not use both of these items simultaneously’.

Indeed, it doesn’t have to be limited to a simple either/or choice between two things.  Maybe it is a ‘choose any one of three’ things – in addition to the iron and the vacuum cleaner, perhaps you also have the hair dryer in the same place and the big placard now says ‘Only use one of these three things at any time’.

You can adjust your instantaneous power requirement down by adopting some of these strategies.

Step Three – Timing Your Energy Needs

Remember that solar power only flows from your panels when the sun is shining.  It will flow at the greatest rate around the middle of the day, when your panels will be most perpendicular to the sun, and at lesser rates before and after that time.

So, for anything that you can conveniently time-shift into the middle of the day, you should do that.  Other things, wherever possible, still try to schedule for daylight hours rather than nighttime so as to take power direct from the solar panels rather than from the batteries.  For example, instead of having a shower, washing your hair and then drying it at night, do that in the morning when the sun is shining and your panels are giving you power for the hair dryer.

It is always better to use energy direct from your panels than to require the energy to detour through a battery system – this keeps things simpler and cheaper, and your batteries will probably fail long before your panels do.

In winter time, cook hot meals before the sun sets.  Do all your ‘chores’ that require appliances during the day, not at night.

Work out the power profile and total energy you need during the daylight hours and do a similar calculation for the night-time hours.  We suggest you consider daylight to start from perhaps 30 minutes after sunrise and to finish perhaps 30 minutes before sunset.  Sure, there’ll be some sunlight and energy flow in the time you’ve omitted, but keep that as ’emergency/bonus’ power.  It won’t be all that much, anyway.

So now you should end up with information for the peak power consumption and total energy needed, day and night, for the three seasons – spring/fall, summer and winter, with splits showing essential power, desirable power, and not necessary power.

Well done.  Let’s now look at what you do with these numbers.

Step Four – Sunlight Hours

Use the resources on this page to see how many sunlight hours a day you can expect at your location for winter, spring/fall, and summer.  Use the monthly PV Solar Radiation (10 km) static maps.

Now you are going to need to start massaging the numbers and making some assumptions.  What you see on these maps are the average daily sun hours.  But we know some days will have more than this, and other days will have less than this.  Do you want to plan your energy supply based on the hope that you never have a bad day (in terms of sunlight)?  That would be foolishly optimistic, wouldn’t it!  You need to be willing to spend some more for additional capacity so as to allow for some days giving you less energy than average.

Depending on the point at where your comfort level starts and stops, we suggest you might choose to reduce the daily hours of sunlight by 50%, to get a moderately pessimistic prediction of how much energy you can get on a cloudy day.  If you wanted to be truly pessimistic, you could reduce by 70%.  (Note there is a balancing factor – how much battery storage you have – in general terms, the greater their energy storage, the less the power capacity you need.)

Anyway, let’s say you reduced the sunlight rating by 50%, and let’s say that your retreat was in an area with 2 – 2.5 hours of sunlight a day in winter.  So you are now looking at 1 – 1.25 hours of sunlight a day; perhaps you’ll round that down and call it 1 hour, or perhaps you’ll take the midpoint and call it 1.125 hours.  Your call.

You also look up sunrise and sunset tables for your location, and see that on the shortest day, you have almost 9 hours between sunrise and sunset, and after taking off 30 minutes from the sunrise and sunset time, that leaves you with 8 hours during which there will be useful power flowing from your solar array.

You’ve also done your figuring, and let’s say you’ve ended up needing 20 kWh of energy for the 24 hr period, of which you can use up 15 kWh during daylight hours and need the other 5 kWh for night-time.  (Note – this is probably a high number, so don’t panic at the figures that follow below.  Most people are likely to come up with a number half this or even less, so all other numbers would halve, too.)

The first thing you now know is that you’ve got 1.125 full sunlight hours to generate 20 kWh of energy.  You need your panels to be rated to give you 17.8 kW of power in full sunlight, plus some extra to allow for inefficiencies and losses in the system – at least 10% extra, and better to go up 25% extra.  We’d suggest a 25 kW system would be the minimum to handle this.

There’s an interesting extra thing you now know, too.  With 1.125 sunlight hours, spread over 8 hours with the sun in the sky, your system will typically be generating power at about 1.125/8 of its full rated power, ie, at about 14% of its 25 kW full power rating – about 3.5 kW.  Sometimes it might go way higher than this, and other times, it will drop down below that.

But what you know, from this, is that you’re not likely to be able to run much more than about 3 kW worth of appliances simultaneously during the eight hours of the day.  If you need more instantaneous power than this, you’ll probably need to increase your system’s capacity beyond 25 kW.  On the other hand, you will have a flood of ‘bonus’ power – instead of it trickling in to your retreat at 3.5 kW, it will be flooding in at 25 kW.  It would be nice to have some ways of using/banking/storing this extra power when it does come in (maybe heating your hot water hotter than normal at such times, maybe using appliances more, maybe slightly overheating or overcooling your retreat, and so on).

You can – if you wish – do the sunlight calculation for the three different seasons and match that against the power requirements for each of the three seasons, but probably you’ll find the critical calculation will be the sunlight available in winter matched against your winter energy needs.

Step Five – Batteries for Overnight

Now to consider the implications of your energy storage needs.

Continuing the example from the previous step, you know that you need to store 5 kWh of energy for night-time, while the sun is down and the solar system isn’t generating any power.  So clearly you need a battery capacity sufficient for this, and with some extra to allow for the power losses between converting from the energy stored in your battery bank to the electricity for your retreat.

Calculating the amount of battery capacity you’ll need is a complicated process, and we discuss it in this article – Using Batteries to Store Electricity.

You need to allow for the fact that not all the energy stored in the battery can be converted back to electricity, and you need to also adjust for the fact that you’ll probably only want to discharge your batteries down to somewhere between 50% and 75% of their rated capacity (depending on the battery type you have) each night, and adjust further for the rate of discharge being, for much of the time, probably faster than the discharge rate used to calculate their capacity (the faster you discharge a battery, the less charge you’ll get from it).

Oh yes, the battery capacity is probably also based on the possibly unrealistic expectation that it is stored at a temperature in the high 70s – and as it gets colder than that, it loses capacity (at 50°, you’ve probably lost 10% of its capacity due to the colder temperature, and at 30° you’d have lost 20%).

So, put all that together, and what do you get?  Some sort of a guess figure – in this case, if I had batteries I was comfortable discharging to 75% of capacity, I’d first say ‘5 kWh is 75% of 6.7 kWh’ and then I’d adjust for the discharge rate used to make the 6.7 kWh capacity, which might require another 20% increase in capacity (up to 8.3 kWh) and then maybe I need to add another 10% for converter and distribution losses (taking us up to 9.3 kWh), and then another 10% for the batteries being stored at less than 77°.

That takes us up to 10.3 kWh, and so I’d probably round that up to 11 kWh.

Note that this means that your earlier 20 kWh of energy figure, based on 5 kWh of battery power, needs to be increased for these losses.  You sort of did that already when you went from 20 kWh to 25 kWh, but if your battery power needs are much larger than in this worked example, you might need to adjust up even further.

Putting it All Together

So, you’ve worked out how much energy you need, and when you need it.  You’ve also worked out the maximum power supply you’ll need, and you’ve done the calculations three times, variously for spring/fall, summer and winter.  Maybe you’ve done them even more times, to adjust for the bare essential minimum of power, some more power that would be desirable, and still more power that would be ‘luxurious’.

You’ve also worked out the hours of effective sunlight you’ll get at the worst points of these three seasons, and adjusted down from there for a ‘worst case’ scenario, and used that to work out the specifications for the solar panels you need and their power generating ability, and how much battery storage you need to see you through each night.

Is that all?

Actually, no.  You probably should now add some extra margin into your figures to allow for truly worst case scenarios – for two or three or four days with much less sun than you were figuring on happening in a row (and, when you think about it, really bad storms can last for several days, can’t they).

You also should include some system redundancy for failures and other unexpected events.

How much more do you need to add to your system?  Please read the next article in this series – ‘How much emergency/reserve capacity does your solar power system need‘ for a detailed discussion on that point.

For now, you’ve worked out that for your retreat, needing 15 kWh of energy during the day and 5 kWh of energy at night, and with winter sunlight averaging 2 – 2.5 hours a day, you should choose a solar system rated at 25kW and a battery bank with 11 kWh of storage.

Please keep reading to see how much further this will need to grow.

Can We Help?

If this is all a bit too much for you, we are happy to consult with you and do most of this all on your behalf, and to walk you through the personal preference/lifestyle choices that we can’t make for you.  Rates are reasonable and start at $250.  Let us know if we can assist.

Mar 212014
 
A split system heat pump for heating and cooling your retreat might be surprisingly practical to consider.

A split system heat pump for heating and cooling your retreat might be surprisingly practical to consider.

One of the basic principles of planning a retreat is to minimize your energy needs, and a key part of that is the design of the retreat so as to make it as well insulated as possible.  This will cut down on your heating energy requirements in the colder months, and should also cut down on your cooling energy requirements in the warmer months.

Well, that’s the theory of it, anyway.  The reality is a bit different.

The thing is that while a well insulated house will slow down the rate at which outside heat comes in to your house, it also traps the heat inside and, well, keeps it there, which can mean that inside temperatures will rise to match the outside temperatures, no matter how extreme it may be outside, and you’ll be forced to ‘give in’ and open up all the doors and windows in the summer months, just to get some air flow, even if of hot ambient air.

You’ll also try to also flush out the hot air in the coolest hours of the night, so you start off each day with as low an indoor temperature as possible, and for the first part of the day, as it inexorably rises, you’ll be moderately comfortable, then when inside and outside temperatures approach the same point, you switch from an all shut up to an all open strategy for the rest of the day.

A related issue may be humidity control, depending on if you’re blessed with a relatively dry climate or cursed with a humid one.

This heating effect is of course more pronounced in summer than winter.  In winter, it is a good thing, but in summer, not so good.  Our bodies are radiating heat all the time (100W – 150W for a typical moderately active adult, less while we sleep, more when doing strenuous physical activity), and all the energy we use indoors eventually ends up as heat, too.  So, depending on your energy consumption each day, you probably have the equivalent of a one bar heater on all day every day, which is why, all year round, your indoors temperature is warmer than outdoors, even before you start adding specific additional heating.

We, ourselves, hate being hot, and productivity studies have shown people become materially less productive whenever temperatures start to climb above 70°.  We also hate trying to sleep in a hot stuffy room, and can confirm from personal experience the additional studies that correlate good or bad sleeping with the ambient room temperature.  We love air conditioning.

On the other hand, air conditioning can consume large amounts of energy.  A typical 110V a/c window unit will run at about 1 watt for every 10 BTU of cooling – a 10,000 BTU unit would draw 1000 watts, although note that its duty cycle – that is, the amount of time it will be on – will be maybe 25% – 50%, so you’re getting an hour of cooling for maybe only 250 – 500 watt hours of energy.  Larger a/c systems, and using higher voltages and/or three-phase power, can be more efficient than this and give you more cooling per Watt hour.

As an interesting additional comment, did you know that because a/c units simply shift heat rather than create cold, they move more heat than the energy they consume.  This has implications for both winter and summer – if you have a heat pump, it will create probably two to three times as much heat per kWh of energy as would a normal resistance heater, depending on the temperature of the outside air.  Cooling units typically ‘suck out’ three or four times as much energy as they consume.

Energy Efficiency Issues

Needless to say, if you are installing a/c at your retreat, you want it to be as energy-efficient as possible.

In the US, a/c systems are given a SEER rating or sometimes an EER rating.  Both are a measure of their energy efficiency – the higher the number, the better.  SEER numbers are higher than EER numbers for the same unit by about 15% (ie something with a 14 EER rating would be the closely similar to something else with a 17 SEER rating.

Normally, when a person buys an a/c unit, they give some passing thought to the SEER rating, but pay more attention to other issues like the cost, the noise level, and so on.  However, for a grid-down situation, where energy is never plentiful and always ‘expensive’ in some form or another, you’ll want to make the SEER rating one of your primary focuses.

Generally, split systems, with a unit outside and a separate unit inside are more efficient than all-in-one units such as are typically installed in window frames.  Split systems can give you SEER ratings into the mid to high 20s; all-in-one units struggle to reach 15.

Heating Too?

As we hinted at obliquely above, if you’ll be needing to use electrical heating in the winter, do consider a heat pump rather than just basic simple resistance heaters, because you’ll get two or three times as much heat from each unit of electrical energy with a heat pump than a regular resistance heater.

The efficiency of a heat pump, for heating purposes, depends on its design and the outside temperatures you’ll likely encounter.  The colder it is outside, the less efficient the heat pump becomes.

There are heat pumps specifically designed to work better in very low outside temperatures, and beyond that, you can also switch from an air-exchange heat pump to one with underground piping, transferring the heat from the warmer ground rather than from the cooler air.  Underground piped systems can become quite a lot more complicated and expensive, so we’d consider those with caution, unless you really need an electrically powered heating solution for your retreat.

In general, we’d hesitate to recommend relying primarily on a solar based electrical heating system, unless you’re so overflowing with solar power that you have plenty spare, even on the coldest and least sunny winter days.  If, for whatever reason, you have no other sources of energy from which to create heat (such as firewood), then maybe you have to use solar, and in such a case, it might be a better and more direct approach to simply install a solar heating system, directly transferring what heat there might be from the sun from outside to inside.

Whereas with cooling, the more sun there is, the more you need cooling, and the more solar power you have available to meet that need, with heating, the equation is the opposite.  The less there is sun, the more you need heating, but the less solar energy you have available, in any form, to use for heat.

But, having said that, we’d probably look at the cost difference between getting a cooling-only a/c system and a dual heat/cool system.  If there’s not a lot of difference in cost, we’d get the dual purpose system, because on the days when we do have surplus solar power, why not save our firewood or other energy sources and use the solar power for our heating needs.

Your A/C Needs are Matched by Your Solar Power Outputs

So, as mentioned in the preceding paragraph, there’s a wonderful thing about solar power that makes it sensible to consider about using your solar power to drive an a/c unit.  The stronger the sun, the higher the temperatures, and, at the same time, the greater the power output from your solar panels.  Okay, so that’s a bit of a simplification – in some areas, it can be hot, humid and horrible, even if there’s little or no sun at all, but in other areas, if the sun is obscured, the temperatures drop.

Our point is simply this.  You’ve probably tailored your solar power system to provide you all the power you need in the winter months with little sunlight.  So, now you’re in the summer months, with more and stronger sun each day, you’ll be getting a lot more energy from your solar setup – maybe even more than you need.  Because of the close relationship between your solar panels generating more ‘bonus’ energy for your use, and the times when you’d most benefit from a/c, it becomes possible to plan to use your a/c only when you have surplus spare power, because those times are also the times you most want your a/c running.

So, if the climate warrants it, go ahead and treat yourself, and fit some a/c to your retreat.

Dec 202013
 
A roof that uses solar energy three ways - solar power panels, solar hot water heating, and a skylight.

A roof that uses solar energy three ways – solar power panels, solar hot water heating, and a skylight.

This is the first part of a series on solar energy.  Please visit our sections on energy in general and solar energy in particular for more related articles.

Every part of our world is defined – and/or constrained – by energy.  Our modern civilization – something you might think of as being all about computers and the internet, or perhaps all about big cities and high-rise buildings, or perhaps jet planes and satellites – has one universal thing at the root of every part of it – energy.

Try and think of any part of your normal life in which energy is not an important part.  Energy makes your home warm in the winter and cool in the summer.  Energy allows you to commute to work.  Energy enables you to work on the 10th or even 100th floor of a building.  Energy is converted into food, is used to transport food to your local supermarket, to store it there, and more energy is used by you to cook the food.  Energy also brings you the water you need, and takes away the sewage you don’t need.

Energy powers your cell phone and computer, and also the internet they connect to.

Energy comes in many forms, of course.  In earliest times, energy was primarily either in the form of our own personal labors, or in the form of fire.  In time, we harnessed additional energy sources – we used animals to magnify our own personal energy outputs, and of course, we started to develop new ways to channel the energy of a fire – the external combustion engine (or, if you prefer, the steam engine) was the breakthrough that allowed for the Industrial Revolution, and subsequently for the settling of the United States, and we know all about the internal combustion engine’s subsequent impacts.

Okay, we will skip the rest of the history of energy development, and simply say, for our purposes, the biggest challenge we have to face in a Level 2 or 3 situation will be the loss of our usual energy sources and the critical energy shortage that will result.  There’ll no longer be gas at the gas station, or electricity in our sockets at home, or other sources of energy in other places, either.  Our lives will fall apart due to the lack of energy to maintain, manage and improve our lives.

So one of the most important preparations we have to make is to identify suitable sources of energy to carry us through any future Level 1, 2 or 3 situation.  A level one situation is relatively trivial, and we won’t dwell on that here, but when we start to think about extended level two and three scenarios, we realize that a supply of spare batteries, some stored petrol, a generator, and an outdoor barbecue with a couple of spare propane tanks isn’t going to take us very far.

Sure, we could grow our fuel dump and instead of 5 or 50 gallons of petrol, look instead at 500 or 5,000 or more gallons, and probably of diesel.  Instead of a couple of standard 5 gallon propane tanks, we could install a below-ground series of 1,000 gallon tanks, even 10,000 gallon tanks.

We’re not saying you shouldn’t do all these things, but these are non-trivial preparations, costing probably tens of thousands of dollars and requiring large storage spaces and possibly needing special permits to store large quantities of fuel.  And, even if none of these challenges are problems to you, there’s still the ultimate problem that you’re merely delaying – no matter how much fuel you store, sooner or later, you will run out, and then what do you do?

So, we preppers are always keen to find renewable energy sources.  The three most common are hydro, wind and solar.

Hydro has very limited applicability for most of us, and wind power is not often a practical consideration either.  Even if you are fortunate to be able to add a wind powered turbine to your retreat, you will need to plan for ongoing repairs and maintenance and the replacement of stressed moving parts from time to time.

Which leaves us with the other common source of renewable energy – solar cells (or, if you prefer, PVs – photo voltaic cells).

If you live in an apartment with no roof and only a small amount of northern facing windows, then you probably can not get much value from solar power.  But if you have a reasonable amount of sky-facing space, and ideally giving you an unobstructed view of the horizon for 150+ degrees from east, rotating down through south and over to west, then solar might be a good choice.

Solar cells continue to drift down in price and to drift up in functionality and efficiency – that is, a given size of solar panel seems able to generate more and more power with each new generation of solar cell technology, while costing less and less.  That’s not to say they are a cost affordable alternative to normal utility provided electricity and natural gas in normal times, but in a Level 2/3 situation, they become an affordable and essential energy source.

Of course, they only work when the sun is shining (well, they’ll still generate power at a diminished rate on cloudy days, but not at all when it is night), so they need to be considered in conjunction with some type of energy storage system (ie, for most of us, batteries).

Perhaps the most appealing part of a solar array is that the solar cell panels have a long lifetime.  They are typically warranted for periods of about 25 years (although the warranties usually allow for some diminution in power output over time, but typically warranted to still generate 80% or more of their initially rated power after 25 years).  With no moving parts, there’s almost nothing to maintain (you need to keep them clean, and their associated battery bank and charging electronics may need maintenance) and almost nothing to go wrong.  Indeed, some solar cells are still working perfectly after 50 years, so even the 25 year warranty might be conservative.

That’s not to say that the panels are (literally) bullet proof, and indeed, a very remote possible source of damage might be hail.  But the slope on your panels will help deflect hail rather than directly absorb its impact, and assuming your hail never gets powerful enough to start smashing roofs and car windshields, your solar cells should be just fine (see this story about one exception to this, but it involved hail the size of tennis balls).

How Much Power Can You Expect?

Clearly, the more sunlight, the more power.  But let’s start off with an assumption – bright sunlight – and then understand how much power we can get from normal bright sunlight.

PV cells these days seem to typically be about 14% – 15% efficient.  Planners work on the basis of the sun providing about 1000 watts of power per square meter on a bright sunny day – an interestingly round figure, but one that is good enough for many planning purposes.  A square meter is about 10.8 square feet, so if you prefer, you can also say the sun provides about 93 watts per square foot.

This means that a PV cell with 15% efficiency will convert that 1000 watts of solar power (per sq m) into 150 watts of electrical power (or about 14 watts psf).

This 15% efficiency is massively better than nothing at all, and the sunlight is free to start with.  New types of solar cell are promised in the future, with higher efficiency ratings (up into the mid twenties and possibly even over 30%), and some are even available now, but they are very much more expensive, and may be more fragile and less long-lived, so for now, you are best advised to stick with traditional technologies.

Now that you understand the typical maximum power output to expect, the next issue becomes how much you’ll actually get in real life.  Nowhere in the US has bright sunlight all day every day, and even those places with high levels of bright sunlight still have a mix of day and night each 24 hour period.

There are some very useful maps created by the National Renewable Energy Laboratory which show you, for your specific part of the country, how many equivalent hours of bright sunlight a day you can expect, both month by month and a summary for the entire year.  Use the PV Solar Radiation (10km) monthly map series for the most accurate and detailed information.

You’ll see that most of the US seems to average, over the course of an entire year, at least 5 kWhrs of sunlight per sq m per day.  Another way of putting that is to simply say ‘at least five hours of bright sun in total, even if spread over more than five hours of daylight’.

Remember to now allow for the 15% or so efficiency in converting this sunlight energy into electrical energy, and be sure to look at winter month figures as well as summer, and you can start to get a feeling for how much you’ll actually get from your solar panels.  If you’re up in American Redoubt territory, you’ll see you’ll be struggling to get much more than 2.5 hours of bright sunlight equivalent each day in winter.  You’ll need a lot of panels to get any appreciable energy in winter in such locations.

In other words, if you have a solar panel setup that is rated for, say, 10 kW, and you’re in a part of the country with 5 hours of sunlight a day, you can expect your 10 kW panel to give you almost 50 kWhrs of power per day.

Needless to say, it is best, when considering how much power to get from your setup, to always be conservative and plan for worst case rather than best case scenarios.  Make sure you understand the methodology being used to quote power outputs and total energy generated from systems, because these are more likely to be best-case scenarios.

Please see our article ‘How Much Solar Power Do You Need’ for a more detailed look not just at power outputs from solar panels, but at the power requirements you are likely to have in your retreat and how they can best be handled via solar power, and, if helpful, our article explaining the difference between power and energy, between watts and watt hours.

Partial Shade Problems

Here’s something we learned ourselves, when trying to install a solar panel system in an area that had trees that would cast partial shade patterns on some of the panels.

You need to have even sunlight on each entire panel, and ideally on the complete multi-panel structure.  If you don’t, you’ll find that a small amount of shade, on a small part of one panel, might be enough to cause the entire panel to lose half its power output, and possible to reduce its power output completely down to zero.  Indeed, and here’s the really surprising thing – a panel that is evenly shaded will probably generate more power than a panel that is half in the shade and the other half in full sunlight.

This is not something you’d guess at (which is why we mention it).  The reason for the disproportionate impact of partial shade is the way the panels are made up of the separate cells within them.  While you probably think of your total solar installation as being a collection of panels, you also need to appreciate that each panel is a collection of individual solar cells.  These solar cells are joined together in an electrical series (and sometimes might have several of these series strings of cells all connected together in parallel), so as to take the voltage generated per cell (usually about 0.5V per cell) and have this combined to create a more useful voltage (the higher the voltage, the less power loss in the wiring and the less thick the wiring needs to be) to actually work with.

If one of the cells in the series of cells all joined together gets less sunlight than the others, then instead of generating some power, it can end up in effect ‘sucking’ power from the other cells, and that may cause safety circuits in the panel to then drop the voltage across all the cells in the string to prevent this from happening.  You can sort of think of it as the difference between a band playing a tune all in time with each other, and the disproportionate chaos that results if even one person in a large band starts playing wrong notes.

This is why you want to have even sunlight on the entire panel, so all the cells within it are working together equally.  Be sure to locate your panels appropriately.

The same problem usually affects multiple panel setups, too.  Normally the panels are also connected in series, and if any one panel (or part of one panel) gets less sun, then all the panels suffer.

Note that new micro-inverter circuitry can reduce some of these impacts, and in general give a better ‘yield’ of usable electricity from a given amount of sunlight on the panels.

Three Types of Solar Power

Now for an interesting extension of the concept of solar ‘power’.  Up to this point, we’ve been considering solar power in terms of photo voltaic cells that turn sunlight into electricity.  But there is at least one scenario where there is a better way of harnessing the sun’s energy.

That is when you want to simply heat something up.  Providing hot water, or heating your home in general, requires a great deal of energy to start with (heating up 10 gallons of water from 45 degrees to 95 degrees to give yourself a five-minute shower with warm/hot water, for example, requires 1.22 kWh of electricity.  Or, to put it another way, a full half charge cycle of a 400 Amp hour 6 Volt battery, just for one five-minute shower.

It is more efficient to heat the water directly through a solar water heater than it is to first generate electricity, then store it in a battery, then subsequently route it to the water heater and convert it to heat.

Solar water heaters typically cost slightly more than half the cost of solar power generators, for a similar power output, and whereas you have additional costs for batteries, etc, with the electric power system, you don’t have these at all for the solar water heating (the hot water becomes your ‘store’ of energy).

Amazingly, solar water heaters can still create hot water for you, even if the outside temperature is very cold.  On a bright clear winter’s day, you might have below freezing temperatures, but still be getting hot water from your solar water heater.

So it is sensible to have both solar (electric) power panels and a solar water heater at your retreat.  That will save you substantially, particularly in terms of batteries, and also gives you the added benefit of two separate power generating systems, albeit both dependent on the sun.

There are also several different styles of solar heating that can be used to heat the air inside a retreat.  The simplest of these is, of course, a glass window.

Requirements for A Simple Installation

Ideally, you’ll design and build a system that gives you full energy independence, and allows you to enjoy an energy intensive lifestyle, even in an extended ‘grid down’ Level 2 or 3 event.

But if you want to start off simply and do a ‘proof of concept’ installation, there’s no reason not to do this.  Anything is better than nothing.  You can buy complete systems that include the panels, some type of mounting system, wiring, distribution panel and an inverter (to convert the power from the DC that comes from the panels to regular 110V 60 Hz AC that your appliances use).  This type of simple system (a ‘grid-tied’ system) works when the sun shines, and doesn’t work when the sun doesn’t shine.

The next step up from that is to add a charge controller and a bank of batteries.  Now your panels can either be directly powering your appliances, or, if there is spare power, can be using the power that is coming in to charge up some batteries.  When the sun goes down, your system automatically switches over and now draws power from the batteries to continue supplying your home with 110V AC power, and automatically switches off when your batteries have used up the optimum amount of their charge.

More sophisticated systems can interface with your regular electric power supply (assuming it is available), so that you first use your own solar power and only after that use the utility company’s power.  Many states allow you to now ‘run your meter backwards’ and sell any surplus solar power to the utility company.  Even more amazingly, some states will require the utility company to pay you more for electricity they buy from you than the cost they charge you for the electricity you buy from them.

But we’re talking simple installations to start.  A simple ‘grid-tied’ system that is capable of generating about 1 kW of power in bright sunlight will probably run you just over $3,000; more if you have someone do the installation for you, and if you add a battery bank to this for off-grid operation, then your cost will probably double.  These costs may qualify for a 30% federal tax credit, and maybe there are state and local programs to further reduce your net cost.  There are lots of specialty stores that sell solar power systems, and of course, you can also browse through Amazon (here is just some of the gear they sell) too.

There is little economy of scale with solar systems.  You may get a lower rate per panel when buying ten or twenty or more panels at a time, and if you are using a single inverter rather than micro inverters for each panel, then there might be some savings there, but in general, if you get a system four times as large as a different system, then the cost will be close to four times as much too.

Our point here is simple.  You can get a fairly basic system for under $5000 and a good system for under $20,000.

Installing a Solar Power System

Ideally you want your panels facing south, on an incline the same number of degrees as your latitude north of the equator.  An easy way to align the bank of panels is to place a stick on them, sticking up perpendicular from the panel surface.  Then at exactly midday (or 1pm during daylight saving hours) align the panel so the stick casts no shadow.

If you can’t get it exactly right, don’t worry.  A 15° misalignment only reduces the power generated by perhaps 3.5%.

Panels are typically mounted on the roof of a structure, but there’s no reason why this must be so other than the good sense of using a mounting structure that is more or less already in place.  If you are mounting panels on a roof, it pays to make sure the roof itself, underneath the panels, has a goodly life remaining in it.  If not, perhaps you should first replace the roof before adding the panels above it.

If you are placing panels on the ground, consider if you need some stand-off height so that winter snow won’t reach up and obscure the bottom of any of the panels.

Wherever you choose to mount your panels, try to keep the cable runs as short as possible from the panels to your retreat’s main electrical supply area.  Longer cable runs mean wasted energy through resistance in the cables.

You’ll want to be able to regularly access the panels for cleaning purposes, so consider that.  If you have a very large array of panels, maybe you want to have some walkways in them to make it easy to reach over and clean dust, dirt, excrement, leaves, moss, etc off the panel surfaces.

The vast majority of solar panels are fixed, but it is possible to get panels that move to maintain an angle more closely perpendicular to the sun.  The sun travels through an arc each day, both horizontally and vertically, meaning that for most of every day, the energy collection from the solar cells is not as efficient as it theoretically could be.  A system that aligns itself to point more closely at the sun can generate more than 20% extra energy per day.

But the added complexity and cost of systems that track the sun, either in a horizontal or vertical axis, or even in both axes, generally seems to argue against their implementation, and they are harder to mount on a rooftop.  It is usually cheaper and easier to take the money that a tracker type system would require, and to spend that money on simply increasing the total array size and number of panels, while at the same time avoiding the added maintenance issues and complications that the tracker units would otherwise require.

So, Why is Solar so Good for Preppers?

Solar is an excellent source of energy for several reasons that appeal to preppers.  It is cost-effective (compared to other renewable energy sources) and can be deployed widely across the country.  It is long-lived and reliable and low maintenance, and once it is installed, requires no special skills to keep operational.  It is also ‘low profile’ – there are no sounds or smells or sights or smoke associated with solar power generation, unlike wind turbines, generators, or fires.

It is easy to add to a retreat, and easy to grow and expand over time.  While it is dependent on sunlight, you can adjust your array size to reflect the best and worst case scenarios for the availability of sunlight at your location.  The less sunlight, the larger your array and the greater your battery reservoir.

When designing a retreat, it is helpful to have an east-west roofline so you can fill the south-facing roof side with solar panels for electricity, and solar heating units for your hot water too.

If you don’t yet have any solar power generating resource, you should definitely get some.