Battery Bank Calculator
Size a battery bank for off-grid solar systems.
Calculate battery capacity needed based on daily energy usage and backup days.
Sizing an off-grid battery bank comes down to one question: how much energy do you use in a day, and how many days must the bank carry you when the sun does not cooperate?
The formula
Battery bank (Ah) = (Daily usage Wh × Days of autonomy) ÷ Depth of Discharge ÷ System voltage
Why depth of discharge is in there. A battery’s rated capacity is not the capacity you may actually use. Draining a lead-acid or AGM battery below about half its charge shortens its life dramatically, so the working convention is a 50% depth of discharge: a 200 Ah lead bank gives you 100 usable Ah. Lithium iron phosphate tolerates far deeper cycling and is normally sized at 80%. That difference is the single biggest lever on this page. The same daily load needs a lead bank 1.6 times larger than a lithium one, which is most of the reason lithium has taken over off-grid installs despite costing more per amp-hour.
Why system voltage matters. Amp-hours are not energy; watt-hours are. The same 10 kWh of storage is 833 Ah at 12V, 417 Ah at 24V, or 208 Ah at 48V. Higher voltage means lower current for the same power, which means thinner cable, smaller fuses and less heat lost in the wiring. Under about 1,000 W of load 12V is fine. Past roughly 3,000 W, 48V stops being optional: at 12V a 3 kW inverter draws 250 amps, and cable that thick costs more than the extra batteries.
Worked example
A cabin using 5,000 Wh a day, wanting 2 days of autonomy, on lithium at 48V.
- Energy to store: 5,000 × 2 = 10,000 Wh
- At 80% depth of discharge: 10,000 ÷ 0.8 = 12,500 Wh of rated capacity
- At 48V: 12,500 ÷ 48 = 260 Ah
The same cabin on lead-acid at 12V needs 10,000 ÷ 0.5 ÷ 12 = 1,667 Ah, which is around eight 220 Ah golf-cart batteries against roughly two 130 Ah lithium modules.
What this figure does not include
Treat the answer as a floor, not a specification. Real installs add margin for three things this page does not model: inverter efficiency, which costs you 8 to 12% between the battery and an AC appliance; temperature, since a lead bank at −10°C delivers perhaps 70% of its rated capacity; and ageing, because every chemistry loses capacity over its life. Adding 20 to 25% on top covers all three for most systems. Size the bank for the winter week you actually get, not the annual average.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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