Battery Bank Size Calculation
To store all of the electrical energy produced by solar panels or wind turbines, we need to install a battery storage system. For smaller installations, such as with one solar panel, a single deep-cycle battery might be enough. However, for larger systems, larger battery banks are needed. Following the simple calculations below will give you an idea as to how big your battery bank system needs to be. As a general rule of thumb, a bigger battery bank will be discharged by a lower percentage than a smaller bank and therefore the batteries will stay in top condition for longer.
STEP 1:
Identify total daily use in Watt-hours (Wh). To do this, for each electrical device you want to use, multiply the power rating by the number of hours a day you expect to use them, then add the numbers together.
e.g. 2000Wh/day
STEP 2:
Decide how many days' backup you would like to build into the system. This is the amount of time your battery bank will support you without solar panels attached. Multiply the usage in Wh/day by this factor.
e.g. 2-days of autonomy : 2000 x 2 = 4000 Wh
STEP 3:
Decide on a target 'maximum depth of discharge' (DoD); the lower the better!!. Divide the result of Step 2 by this value (decimalised).
e.g. 40% DoD: 4000 / 0.4 = 10000 Wh
STEP 2:
The storage efficiency of all batteries is closely connected with temperature and we need to account for this in our calculations. Compensate for ambient temperature by selecting the multiplier corresponding to the lowest average temperature your batteries will be exposed to. Multiply result from Step 3 by this factor.
Temp. °F |
Temp. °C |
Factor |
|---|---|---|
80 |
26.7 |
1.00 |
70 |
21.1 |
1.04 |
60 |
15.6 |
1.11 |
50 |
10.0 |
1.19 |
40 |
4.4 |
1.30 |
30 |
-1.1 |
1.40 |
20 |
-6.7 |
1.59 |
e.g. 21oC = 1.04 x 10000 = 10400 Wh
STEP 5:
Wiring up the battery bank to change the voltage from 12V to 24V or 48V is a useful way of reducing the size of the load current. Divide result from Step 4 by the battery bank voltage to get the minimum Amp-hour (Ah) capacity of your battery bank.
e.g. 10000 / 24 = 417Ah
STEP 6:
Work out how many batteries you need based on the Ah capacity rating of your chosen battery model.
e.g. using a 100Ah battery for a 400Ah battery bank:
12V = 400 / 100 = 4 batteries
24V = 400 / 100 * 2 = 8 batteries
48V = 400 / 100 * 4 = 16 batteries
For details about how to wire up different battery bank configurations, please read our advice section: Battery Bank Wiring.
