11 May Solar and Battery Sizing
What Size Solar Panel and Battery Do I Need?
This is one of the most common questions we get at RV Magic. What size solar panel and battery do I need? Or variations of I have x amount of solar what size batteries do I need. So let’s try and explain how to work this out.
The best way to explain this in the way of an example.
John and Jill have a weekender caravan, they have a basic setup and require power to run 20w of LED lights in the evenings.
They wish to run a load of 20W (the LEDS).
The lights will be run for 5 hours per day.
They get an average of 4 hours of peak sun each day (this will vary on your location and the time of year).
We want to size the system large enough to handle 3.5 rainy/cloudy days without sun.
Lets assume they want to use AGM style batteries and lets say they will be 12v ones.
Now to the actual calculations you need. To start with we work out what sized batteries we need and then what size solar panels we need to charge them.
Now to work out how many watt-hours John and Jill require we need to multiply our load by the hours it will be running so in our example it is 20w x 5 hours which equals 100 Watt-Hours.
Next we can work out what size batteries we will need to run this 100 Watt-hours for our 3.5 rainy/cloudy days. We can calculate the Amp-hours required by dividing our Watt-hours (from above) by our battery voltage (12V) and then multiple it by our bad day allowance (4).
So 100 Watt-hours / 12V = 8.3 x 3.5 bad days = 29 Amp-Hours with a AGM battery you only want to discharge the battery by say 30% (leaving 70%) to prolong its life. If you start running your battery down lower you drastically reduce it’s lifespan and they are likely your biggest off-grid investment, so look after them. So our 29 Amp-hours now becomes a touch under 100 Amp-hours if we are only using 30% of our battery. So in this case a 100 Amp-Hour battery would be a good fit. Something like this.
Now we can calculate how much solar John and Jill will need to replace the power used by their lights. So using our example above we had a daily use of 100 Watt hours and our average sunshine in this example is 4 hours. To fully recharge our battery after 1 evening’s light usage we will need a 25w solar panel – 100 Watt Hours / 4 hours = 25w. This is obviously a very small panel as this example all we are running is LED lights, most of us will run a lot more than that so it is important to total and calculate everything use. When it comes to solar I always recommend to fit as much as you can afford, this equates to less discharge of your batteries (aiding their lifespan) and the ability to charge more on cloudy/rainy days.
Always allow a bit extra over your calculations, real world situations are never textbook and having some extra solar and battery storage will be hugely beneficial. Especially on those cold cloudy days when you just want to watch a little tv or read a book under some lights.