Off-grid battery bank calculator
Sizing a bank is a question of how many cloudy days you want to survive, and how deeply you are willing to discharge.
Wh
%
%
V
Bank size
14.118kWh
- Capacity at the system voltage
- 294.1 Ah
- Nominal energy required
- 14 118 Wh
- Usable energy
- 7.059 kWh
- Roughly, as lead-acid At about 30 kg per nominal kWh.
- 424
- Roughly, as lithium iron phosphate At about 6.5 kg per nominal kWh.
- 92
Show the working
- Daily consumption = 3000
- Days of autonomy = 2
- Depth of discharge = 50
- Round-trip efficiency = 85
- System voltage = 48
- Nominal energy required = 3000 * 2 / (50/100) / (85/100) = 14 117.6
- Capacity at the system voltage = 14 117.6 / 48 = 294.118
- Bank size = 14 117.6 / 1000 = 14.1176
- Usable energy = 14.1176 * 50/100 = 7.058 82
- Roughly, as lead-acid = 14.1176 * 30 = 423.529
- Roughly, as lithium iron phosphate = 14.1176 * 6.5 = 91.7647
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- bank energy = daily load x days / (depth of discharge x round-trip efficiency)
Deep-cycling lead-acid past 50% shortens its life sharply; LiFePO4 tolerates 80 to 90% routinely.