How Much Extra Emergency/Reserve Capacity Does Your Solar Power System Need?

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.


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.


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.

5 Replies to “How Much Extra Emergency/Reserve Capacity Does Your Solar Power System Need?”

  1. Stew

    Good thoughts, David. But you left out the Charge Rate. Assuming one is using flooded cell batteries, the rule of thumb is to charge the bank at Capacity/10, give or take, in order to stir the acid and keep it from stratifying.

    So, I think the answer to your question is, more batteries AND panels. Size the bank to carry you through your worst case, cloudy-day spell, and have the panel capacity to top the bank off at the proper rate when you get another good, sunny day.

    One other thought for too low a charge rate would be to go on an energy fast when you get some sunny days. Let all the panel energy go into the bank and fully charge it. Then, put your charge controller into equalize mode to stir the electrolyte.

    • David Spero

      Hi, Stew

      Full marks for your comments about charge rates and energy fasting (thank you). Batteries really can be a hassle to maintain well, and all the more so when the charge rate is highly variable, as is the case with solar panels.

  2. Roger

    I should count myself lucky I guess. I live in the Mojave Desert and we get probably 360 days of sunshine!

    A while back I purchased a complete solar kit from Harbor Freight to help me learn about solar and to charge the dual batteries on our travel trailer so I wouldn’t have to hear a generator running while camping. I’m thinking of using those solar panels and purchase a few more to run everything in our living room, lights, tv satellite tv system, flat screen tv. and etc. as one more learning step. I’ll probably run a couple of ham radios occasionally also.

    I noticed the battery box in the picture above was insulated. Wouldn’t that suffice for milder winter climates?

    • David Spero

      Hi, Roger

      It is rare to read a person living in the Mojave Desert calling themselves lucky, but to each his own, I guess!

      Yes, you are completely correct. Where you are, leave your batteries outside, and feel good about doing so. But for those of us ‘not so lucky’, where snow is a familiar part of our winter routine, please keep your batteries as warm as you keep yourselves.

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