Battery Capacity Calculator
Sizes a battery bank from the load and the runtime you want. Accounts for the fraction of the pack you can actually use and for inverter losses.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
usable Wh = load W × hours ÷ inverter efficiency nameplate Wh = usable Wh ÷ depth of discharge Ah = nameplate Wh ÷ system voltage
The two divisions are what separate a real answer from a naive one. A 100 Ah lead-acid battery does not give you 100 Ah — half of it is off limits if you want the battery to last — and the inverter takes its cut before the load sees anything.
What to watch out for
Three things people leave out, each of which makes the bank too small:
- Depth of discharge. Flatten a lead-acid battery regularly and it dies in a year. Half its rated capacity is the working figure; lithium tolerates 80–90%.
- Inverter efficiency. Typically 85–93%, and it is worse at light loads. A DC load skips this entirely.
- Peukert's effect. Lead-acid capacity falls as the discharge rate rises — a 100 Ah battery drained in one hour may deliver only 60 Ah. Lithium is largely immune.
Temperature matters too: capacity drops noticeably below about 10 °C, and most lithium packs will not accept charge below freezing.
Frequently Asked Questions
What size battery do I need to run 200 W for 8 hours?
About 1,778 Wh of usable energy after a 90% efficient inverter. On LiFePO₄ at 90% depth of discharge that is a 1,975 Wh pack — roughly 165 Ah at 12 V. On lead-acid at 50% it would be nearly 300 Ah.
How do I convert watt-hours to amp-hours?
Divide watt-hours by the system voltage. 1,200 Wh at 12 V is 100 Ah; the same energy at 48 V is only 25 Ah, which is why higher-voltage systems use much thinner cable.
Why can I not use the full rated capacity?
Because discharging a battery completely shortens its life dramatically. Lead-acid is conventionally limited to 50%, lithium to 80–90%. The rated figure is what the battery holds, not what you should take out of it.