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How to size a battery


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This is something I find a lot of people get wring in the industry and thus their batteries don't last.


The following needs to be know before starting to size:

Voltage of the battery bank, 12, 24 or 48V are the common PV system voltages

Autonomy on your battery bank (be it min, hours, days), taking into account the C rating you are working at, ie the number of hours

Ah or Wh of your load


If you have the Ah make sure it is with the correct voltage of your battery bank, if you have Wh divide it by your battery bank's voltage.


Once you have worked out your Ah of your load, and the ampere hour is per day, multiply it by the days autonomy you want on your system.


Now that you have that value, divide it by the % depth of discharge (%DOD) of the battery you want to use, remember the smaller the %DOD, the more number of cycles you will get out of your battery, the longer your battery life. Also take into account that if you sized for say 3 days autonomy your battery will only get to the %DOD after three days.


Now go look on the battery datasheet for a battery that will give you the recommended Ah at the C rating that you selected above.


Lets do a example:

My system voltage is 12V

My system consumes: 15000Wh per day

I want 4 days autonomy (C96)

I will use the Willard RT range of batteries, see attached brochure


Thus my load is 15000Wh/12V = 1250Ah

The batteries can take 80% DOD, thus 1250Ah/0.8= 1562.50Ah


So I need 1562.50Ah battery bank at C96, 12V system voltage

On the datasheet C100 is the closest range.

There is no battery that can supply the Ah needed all at once, thus divide the Ah by 2, 1562.50Ah/2 = 781.25Ah.


The RT 21 has 730Ah at C100 so that is to small

The RT23 has 803Ah at C100 so that is to big and will cover the need of 781.25Ah


Thus you have 2 banks of 12V in parallel on the system, giving a combined 1606Ah which will cover your need.

And remember these are two volt cells, so you need six of them in series per bank.

Thus 12 batteries in total.









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