Battery Bank Size Calulations

solar battery bank

Solar or wind energy needs to be stored somewhere and typically this is done using deep-cycle leisure batteries. For many installations of one or two solar panels, one large battery has enough storage capacity, but for larger systems it may be necessary to connect multiple batteries to create a 'battery bank'. To work out how much battery storage capacity you need, you need to first know how much power your system will be drawing every day and then follow the simple calculations below. A general rule for all batteries is that the less they are discharged, the longer their service life. 

Please be aware that when using an inverter, it should be connected directly to the battery bank and the battery bank must be of sufficient size to be able to cope with the potentially high current draw of the inverter. Theoretically, a current of 1A on the AC side of the inverter can become 20A on the DC side (A = V x W).

Inverter Size
Typical Battery Bank Size (12V)
1000W
420Ah
1500W
540Ah
2000W
750Ah
2500W
1000Ah

STEP 1:

Calculate your daily power consumption in Watt-hours (Wh) by adding together the power consumption of each electrical device that you will use. You can calculate the daily power consumption by multiplying the wattage rating of the device by the number of hours it will be used for. If the device does not declare the power consumption in Watts, multiply the current in Amps (A) by the operating voltage (V).

e.g. 4 x 100W lights used for 5-hours a day = 2000Wh/day

STEP 2:

Ideally, a battery bank should be able to supply you with power, even if there is a problem with the solar panels or charge controller. You should now decide how many days' of backup power you would like and multiply the power consumption figure from step one by the number of backup days.

e.g. 2-days' backup : 2000 x 2 = 4000 Wh

STEP 3:

As mentioned previously, regardless of battery type and cost, the longest service life will be achieved by discharging them as little as possible.

Decide on a calculated 'maximum depth of discharge' (DoD) whereby the lower the better and divide the result from Step 2 by this (decimalised).

e.g. 50% DoD: 4000 / 0.5 = 8000 Wh

STEP 4:

All batteries are less efficient at lower temperatures and we can compensate for this in our calculation. Choose the factor that corresponds to the lowest average temperature your batteries will have to work in. Multiply result from Step 3 by this number.

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 8000 = 8320Wh

STEP 5:

Depending on the voltage of your electrical system, you may need to connect batteries together to create a bank at 12, 24 or 48V. Using a higher voltage is also a useful way of reducing voltage loss over longer distances or reducing the size of charge controller you need. In order to work out the minimum capacity of your battery or battery bank, divide the result from Step 4 by the desired voltage.

e.g. 8320 / 24 = 347Ah

STEP 6:

Finally, identify how many batteries you need. Ideally, we try to stay within 5% of the calculated size required, so based on the bank voltage and the target Ah capacity.

e.g. 110Ah (12V) leisure batteries for a 330Ah 24V battery bank:
24V = 330 / 110 * 2 = 6 batteries

If you wanted to create a 330Ah battery bank at 12V or 48V, you would need 3 and 12 batteries respectively:

12V = 330 / 110 = 3 batteries
48V = 330 / 110 * 4 = 12 batteries