How Much Solar Power Generating Capacity Do You Need?

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.

5 Replies to “How Much Solar Power Generating Capacity Do You Need?”

  1. Xaun Loc

    And, don’t forget that in most of the country, your rooftop solar panels may spend several days at a time covered by a layer of snow!

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