Instructables
Picture of How to Size Your Off-Grid Solar Batteries
If you are designing a solar electricity system and don't have access to the grid, you are going to have to deal with batteries. When you start looking at solar batteries you are going to encounter a little math. Fortunately,  SolarTown is here to guide you through the calculations!

The general goal when designing an off-grid battery bank is to get a system that is big enough to supply all your needs for a few cloudy days, but is also small enough to be charged by your panels. Before you get started here you may want to check out this article. It serves as a good introduction to the solar batteries I'll be talking about:

http://www.solartown.com/learning/solar-panels/solar-battery-storage-systems-if-you-cant-tell-your-agm-from-your-gel

Alright let's get started! Here are the steps to sizing your off-grid system.
 
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Step 1: Calculating Your Amp-hour Needs

Picture of Calculating Your Amp-hour Needs
1.  Inverter size
To determine the inverter size we must find the peak load or maximum wattage of your home. This is found by adding up the wattage of the appliances and devices that could be run at the same time. Include everything from microwaves and lights to computers and clocks. The sum will tell you which inverter size you need.

Example: A room has two 60 watt light bulb and a 300 watt desktop computer. The inverter size is 60 x 2 + 300 = 420 watts

2.  Daily energy use
Next find the energy used in a day. Figure out how long each electronic device will be run in hours during a day. Multiply the wattage of each device by its run-time to get the energy in watt-hours per day. Add up all the watt-hour values to get a total for your home. This estimate is likely too low as there will be efficiency loses. To get a very rough idea of the real value with system loses, multiply by 1.5. This will help account for decreasing performance when temperature increases.

Example: Light bulbs run for 5 hours a day. Computer runs for 2 hours a day. 120 x 5 + 300 x 2 = 1200 watt-hours. 1200 x 1.5 = 1800 watt-hours

3.  Days of autonomy
Now decide how many days worth of energy you want to store in your battery bank. Generally this is anywhere from two to five.

4.  Battery bank capacity
Finally we can calculate the minimum battery AH capacity. Take the watt-hours per day and multiply them by the number you decided upon in 3. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage.

Example:  You want the battery bank to last three days without recharging and that you use 1.8 kwh per day. As 1.8 x 3 x 2 = 10.8kwh, this is the energy we need from the batteries. Converting this to AH we have to divide by the voltage of your system. This can be 12, 24 or 48 for commercial application. If we choose to use 48V, the minimum AH capacity is then 10 800/48 = 225 AH. Now if you divide by your battery's rating you find the number of batteries you must use.

Great help, I have installed a 2 x 235 watt Solar panels and they are up and running

It was easy to follow and calculate all the energy needed.

keep it up. Pretoria South africa

kmyers619 days ago

200 watts a hour / 24hrs and end up needing, 24 batteries that are 150 amp just to run a small air pump, sorry something wrong in this calculations, these are truck batteries at $100 each don't add up that worth it and 4 solar panel 2x75 and 2x80

Can buy a lot of fuel for small generator for this price of batteries

Starting to sound like a yacht price of sails mast rigging "need replacing every ten years max if lucky" etc ropes,pulleys etc. ends up better off to buy fuel get there lot easier.

Please guide me
I have 72,000 watts hours a day consumption, how much solar panels should i installed, here we have avg 10 hours solar can charge battreys, also i want to use cells instade of batteries, 48V, please tell me how much solar panels and 48v ah cell capacity required
billsmile2 months ago

how calculate watts for the solar panel and charger controller.

Thanks! I could never figure this out. This helps me a lot!

tylerpittson5 months ago

If I have two 265w panels but their maximum output current is 9A do I still need a 60A charge controller? According to what you are saying it would be 530w/12v = 44A

dontno6 months ago

Hey Jon

Thanx for the info for battery sizing. I'm in the planning stages of solar powering my work shed and running a pool pump. This will help greatly help out.

mist42nz10 months ago

400W x 24 hrs = 9.6kW. 9600 * 2days = 19200kW.19200 /50% = 38400kW.
38400/24 v = 1600Ah. 130Ah SCS225 = 28 batterys? sounds pretty expensive/brutal for a basic computer....

mganpate2 years ago
great job ... .. we are interested thanks
MacOSJoey2 years ago
Thanks! Great article! I'm preparing to buy another battery and I found your instructable verey informative and decision-making.
sopranosp2 years ago
Great article, I am very interested in matters of Solar and wind energy
nanosec122 years ago
A lot of great information, but did you really need to PLUG your website in 3 of the 4 steps?
Schmidty162 years ago
looks good