What size solar panel to charge 100ah battery?
If you’ve got a 100Ah battery, and you want to know what size solar panel is needed to charge it, this post goes into everything you need to know. In actual fact, the battery size is somewhat irrelevant, and the only thing that really matters is that you’re replenishing the amount of power drawn each day, but we’ll get into that below.
Please, before you buy any 12V solar panels, check the size against their specified output, as so many are falsely advertised.
There’s a few things you want to check and get right though, or you can do do damage:
Maximum charge rates
It is possible to run ‘too much’ solar into a battery, and knowing how much charge your battery will accept is a good place to start. If you look the battery up online (model and manufacturer), you should be able to find figures that specify the maximum charge rate. If it’s a good quality lithium battery that might be around 100 amps, with lower quality ones around 50 amps, and a lead acid battery around 10 – 20 amps.
With this figure, you’ll know the theoretical maximum solar panel size. Multiply your maximum charge rate by 13 volts, and you’ll get the maximum panel size in watts. Now, your solar controller should reduce the charge down to a suitable level anyway, but having 500W of solar to recharge a 100Ah battery would be overkill in any way you wanted to look at it.
What power are you consuming?
Far more important than the above though, is how much power you’re actually using. If you are just drawing power for a few LED lights and a water pump, you are going to get away with a lot less solar than someone running a freezer, along with an inverter for a coffee machine.
Each day, you want to replace the power that you consume, and have some wiggle room. You can easily work out your consumption either by using a watt meter, or by calculating it, or having a shunt in place.
For example, if you consume 50 amps in a 24 hour period, you need to replace that in the number of hours of sunlight that you get the next day.
How much sunlight do you get?
You might think that because the sun rises at 7AM and sets at 6PM that you get 11 hours of sunlight, and whilst this could be technically correct, the usable amount when it comes to solar panels is much less than this. When the sun is shining directly on the solar panels, they’ll generate the most, but this doesn’t happen immediately, once the sun is up, or late in the afternoon when the sun is down low.
A good consideration is generally 6 hours of decent sunlight, so you underestimate your generation rather than over estimate it, and need more.
More solar is better than less
Ultimately, you’re generally better off with more solar than less. You will never complain when you’ve got too much solar, but struggling to charge your batteries every day gets old real quickly. Get a system that’s larger than you need, and you’ll be laughing.
An example
Lets say you have a 12V fridge, water pump, diesel heater and lights, and every day you consume 55Ah of battery. You need to replenish that entirely the next day, so you take 55Ah and divide it by 6 hours of sun, giving you just over 9A. A good 200W solar panel will do a bit over that, and would recharge your battery comfortably every day as long as the sun is shining properly.
Once it gets overcast things change real quick, which is where an oversized array helps.
If you want a real short and simple guide, double the watts for the Ah, so a 100Ah battery should have about 200W of solar, but if you’re using lithium batteries and using a fair bit of power you can extend this a bit further, to 300 or even 400W.
So, hopefully this helps you understand how much solar you need to charge a 100Ah battery. If you have any questions, leave a comment below and I’ll get back to you!
