The size of solar panels and the size of batteries are determined pretty much independently of each other, but because they work together in any 12Volt system, if we get one wrong then it will probably affect the other. Also, if you’ve already got a 12Volt system but are unsure whether to increase your battery size or solar size, using the examples here and in How much Battery? should sort that out pretty quickly.
But before we can work out how much Solar we’re going to need, there is one key figure we need know.
To get to the how-much question for Solar (and for Battery) we need to back up a bit and ask how much energy I’m going to use each day. The more I use, the more Solar I need, so my daily usage has to be our starting point. For this you can also refer to the article on How much power does 12Volt stuff use?
Totalling up your daily usage is the hard part, but it’s also the key to getting the sizing right. So don’t try and skip this step – it takes a bit of time but it’s worth it!
To give us an example to work with for our solar calcs, we will use a small setup consisting of a 40 litre fridge, a TV and a strip of LED lights. This is the same example used in How much Power does 12Volt stuff use? and we worked out that it will take 45Ah per day to run the system, assuming winter conditions for both the fridge and solar.
Now that we know how much we’re using, we have to work out how much input we can get from the sun, and that’s going to depend on whether you’re in Darwin or Adelaide, Hobart or Cairns. Fortunately the Bureau of Meteorology (BoM) has some excellent climate data on tap, so we can use that. I’ve summarised some of that for typical towns around Australia in the article on Solar Insolation[*] in Australia so I’ll assume you know what I’m talking about in the next para.
For our example let’s take Perth’s weather conditions – for no other reason than I live there. BoM’s figures show that on average we only get 2.6 hours of peak sun in June and 8.3 hours in December, with an annual average of 5.4 hours. I always go for the winter hours – that way I’m doing a worst-case scenario and I know my system will work no matter what time of year I go travelling. If your usage of solar is very specific e.g. a summer-only beach shack, you could adjust the hours a bit.
Now it’s time to look at those panels that are going to supply the daily power that we are using. So in this example we need to put back at least 45Ah into our batteries plus a bit extra for losses in the battery, cables, etc. So on a winter’s day in June we have 2.6 hours of peak sun, so the current we need from the panels is (45Ah/2.6hrs) = 17.31 Amps. Now 12Volt panels typically have a peak voltage (Vpk) of around 17 volts, and that’s the voltage at which their power (Watts) is specified. So to get the Wattage of the panels we need, just multiply 17Volts by 17.31 Amps and you should get 294 Watts. This is the solar power you need to keep your 12Volt system going reliably in worst-case winter conditions.
The Solar Calculator can work all this out for you if you like. Just put in the solar hours for your location, and the daily usage in Ah. It will then spit out a figure in Watts which is the minimum size of panel we need to support the 12Volt equipment we have.
The above figure of course assumes that solar is your only source of power. If you're at a beachside weekender, you might have some wind power, or maybe a gennie that has to run a few hours a day anyway. In that case you can start reducing the amount of solar.
If you have the space, it's always best to stick with this figure, or even go a little bigger – things always grow, especially with 12Volt stuff. Having too much solar is never a problem, it's having too little that causes those little "disagreements" we all have – and who wants that when you're on holiday?!
There's a specific blog post on Solar and Regulators if you're wanting to understand it all a bit better, and if you're wanting to move your solar power around, you can have a look at the one on Portable Solar Panels too.
If you have a vehicle with a dual-battery system which charges the battery while you're driving along, then you can again reduce the solar. But remember that it's only charging while you're driving, so if you plan to drive to the spot and stay put for a week or so, then solar is going to be your only source of power once you park up.
However if the dual-battery system is in a motorhome and you're unlikely to park up for more than a night or two at a time, then the batteries will be kept charged most of the time from the alternator. In this case you could reduce the amount of solar quite appreciably.
There are times when the space available for solar panels is limited – this is typically the case for small boats for example. In that case it's simple – just fill the available space with as much solar as will fit, and in that case an MPPT regulator might be worth the extra cost, just to squeeze the last drop of power out of those panels.
[*] Insolation is just a long word meaning the power-per-area received on a horizontal surface on planet Earth.