Off-Grid Battery Bank Calculator
Sizes an off-grid battery bank from daily consumption and how many days it must run without charging. Accounts for inverter losses, the depth of discharge your chemistry allows, and cold-weather capacity loss.
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The formula
bank kWh = (daily kWh ÷ inverter efficiency) × days of autonomy ÷ (depth of discharge × temperature factor)
The bank has to be bigger than the energy you use, three times over: the inverter wastes some, you must not fully discharge it, and cold weather takes more. Those three factors together typically double the nominal size.
What to watch out for
Three multipliers separate "energy used" from "battery bought":
- Inverter efficiency. Every AC watt costs about 1.09 DC watts. A 92% inverter adds 9% to the bank before anything else.
- Depth of discharge. Lead-acid dies quickly below 50%; LiFePO₄ happily uses 90%. This is the single biggest difference between chemistries and the reason lithium is cheaper per usable kWh despite costing more per nominal kWh.
- Temperature. Lead-acid loses roughly 20% near freezing. Lithium keeps capacity but must not be charged below 0 °C without a heater — a real design constraint, not a derate.
Days of autonomy is a judgement call: 1–2 days if you have a generator, 3–5 if the bank is the only backup. Every extra day is a linear cost increase, which is why most off-grid systems pair 2 days with a generator rather than 5 without.
Frequently Asked Questions
How big a battery bank do I need for off-grid?
For 10 kWh a day with 2 days of autonomy on LiFePO₄ at 90% depth of discharge and a 92% inverter, about 24 kWh — roughly 500 Ah at 48 V.
What depth of discharge should I use?
90% for LiFePO₄, 80% for other lithium, and 50% for lead-acid of any kind. Going deeper on lead-acid shortens its life dramatically.
How many days of autonomy do I need?
1–2 days if a generator can cover a bad stretch, 3–5 if not. More than 5 rarely pays — the bank sits unused most of the year.
Should I use 12 V, 24 V or 48 V?
12 V for small systems under about 1 kW, 24 V up to 3 kW, 48 V above that. Higher voltage means lower current for the same power, so thinner cable and smaller losses.