What Size Solar Panel and Battery Do I Need?
I (Hadley) do a little work with solar powered installations. Lately I've had a few people ask how to work out what size solar panel they need to run some electronics so I thought I would take the time to note down how to calculate it here. Solar dimensioning is pretty straight forward once you know what equations you need to work out. One thing you'll probably notice is that you need a lot bigger solar panels and batteries than you think, especially if you want to run something 24 hours a day.
First for this theoretical example we'll make a few assumptions.
- The load we want to run is 10W
- We want to run the load 24 hours a day
- We get an average of 4 hours of peak sun each day.
- We want to size the system large enough to handle four rainy/cloudy days without sun.
- We'll be using lead acid (car) batteries.
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.
Lets work out the Watt-Hours we'll be using, our load is 10W and it's running all day and night so: 24 hours 10 x 24 = 240 Watt-Hours
Once we have the Watt-Hours and the type of battery we're using we can work out the Amp-Hours, 240 Watt-Hours divided by 12 Volts: 240 / 12 = 20 Amp-Hours. That's only for one day though and we wanted to withstand four days of bad weather (autonomy). Let's multiply: 20 x 4 = 80 Amp-Hours
With a lead acid battery you really only want to discharge the battery to 50% at the most, any more and the battery will wear out quickly. So double the Amp-Hours needed and we get a battery size of 160 Amp-Hours - quite a lot bigger than you would think, also that's quite a big battery, it will be about 30kg!
Now that we have our battery size we can move on to working out what size solar panel we need to charge it. Remember from our first calculation three paragraphs above our daily use is 240 Watt-Hours. Our assumed daily sunshine is 4 hours. To recharge the battery for one days worth of usage in one day charging, we need a solar panel that is: 240 Watt-Hours / 4 hours = 60 Watts.
The charging system isn't 100% efficient, a good rule of thumb is to allow for 20% losses. You might also want to add some solar panel capacity to allow for a quicker recovery from extended periods of bad weather, perhaps another 20%. So your panel choice may by up to: 60 Watts x 140% = 84 Watts. A standard panel sizes are 60W and 90W, depending on your exact usage and the level of autonomy you want somewhere between those would be a good choice.
I hope that gives some insight into how you go about sizing up a solar and battery system. There are some web based solar calculators out there which may do some of this work for you. Have a search around the web. Just beware that a lot of them aren't fantastic quality and it's not all that difficult to do the calculations yourself on a plain old calculator.
To calculate the actual sun hours for places around New Zealand, including the differences between summer and winter, check out SolarView from the good folks at NIWA
While we don't sell large solar system components talked about here we do have a range of small solar modules and components which you might like to check out. If you've got any neat projects you're doing please do let us know in the comments below. Cheers!
Got a little lost with my calculations, as I would need quite a substantial system.
Max monthly usage is about 1660kWh in June, with a central a/c running 24/7 at 19kW(max rating), I don't think it's doable.
As I would not be able to generate that amount of power to supply a huge battery bank?
I was looking at running the panels into a PPT controller and the charging at 48V into a battery bank but don't think it's viable. Crazy prices for batteries that would never pay it's self off in my lifetime.
Brett
Often running the calculations helps you to discover that what you're wanting to do is either impossible or ridiculously expensive. I would say that your calculations are right and it's just not something that makes sense to try and do.
In that sort of situation it's probably better to look at other forms of heating/cooling.
Hadley
Thanks for an easily understandable summary.
No worries, glad it was helpful.
Cheers
Since your "allow for 20% losses" is just a rule of thumb, it doesn't really matter, but if you want to allow for 20% losses you need to multiply by 1.25 not 1.20 (1.25 = 1/0.8).
Please tell me the battery size I need and the panel size?
Desperate for some knowledgable help.
Regards
Lola
You can use the calculations above to work out what battery and panel size you need.
Regards,
Hadley
Your system will need to provide 51 Ah per day, so to ensure you don't over-discharge batteries, you'll need a total capacity of 102 Ah per day. Let's say you get 40 Ah batteries. Then you'll need 12 batteries (3 parallel sets of 4 wired in series). Each set of 4 in series will give you the 48 V supply you need and will contribute 40 Ah, 3 of those sets in parallel will contribute 40 Ah each giving 120 Ah total. Now you need to charge them, so, if you get 6 hours sun every day (no days off) you'll need about 22 solar panels (5 sets of 4 panels), to provide sufficient charge to refresh the batteries if the sun shines 6 hours every day.
we have installed 12 watt solar light with 40ah battery and 75 watt solar panel , we are not getting proper backup for 12 hours so please guide us what we will do,
my e mail address is [email protected]