Battery runtime calculator
A battery stores energy: volts × amp-hours = watt-hours. Take off what you are allowed to use and what the inverter wastes, divide by the load, and you have the hours. Or work it backwards to size the bank.
Straight-line arithmetic on the figures you type. Real banks deliver less when cold, old, or (lead-acid) discharged fast.
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How it works
- Wh = V × Ahenergy stored
- hours = Wh × DoD × η ÷ Wruntime
- Ah = W × hours ÷ (V × DoD × η)bank size needed
Amp-hours on their own do not say how much energy a battery holds — a 100 Ah battery at 24 V holds twice what one at 12 V does. Multiply by the voltage to get watt-hours, the same unit the load uses. Then take only the share you are allowed to use (depth of discharge) and what survives the inverter (efficiency), and divide by the load's watts.
Worked examples
A sump pump backup
- 12 V × 100 Ah = 1,200 Wh stored.
- Lead-acid, planned at 50 %: 600 Wh usable. Through a 90 % inverter: 540 Wh reaches the load.
- A 150 W average load: 540 ÷ 150 = 3.6 hours.
- The battery supplies 150 ÷ 0.9 ÷ 12 ≈ 13.9 A while it runs.
Sizing a bank for a night
- 60 W of lighting and a router for 10 hours = 600 Wh at the load.
- On a 24 V lithium bank at 80 %, through a 92 % inverter: 600 ÷ (24 × 0.8 × 0.92) = 34 Ah.
- Round up to the next size the maker sells, and leave margin for the battery ageing.
Questions people ask
What depth of discharge should I use?
The one the battery maker publishes for the life you want. As a rough habit people plan lead-acid banks around half their rating and lithium (LiFePO4) banks around 80 to 90 %, but the datasheet is the number to type. Drain a battery deeper than its maker allows and it wears out quickly.
Why is my runtime shorter than the calculator says?
Three usual reasons: the load draws more than its label (a motor starting, a heater cycling), the battery is cold or old, or it is lead-acid discharged fast — at high current a lead-acid battery delivers noticeably less than its rated amp-hours (the Peukert effect). Use the rating at the discharge rate closest to yours if the datasheet gives it.
Does a 12 V and a 24 V bank of the same Ah last the same?
No — the 24 V bank stores twice the energy. Two 12 V 100 Ah batteries in series make 24 V 100 Ah (2,400 Wh); in parallel they make 12 V 200 Ah (also 2,400 Wh). Watt-hours are what run the load; type the bank’s total volts and Ah.
Do I need the inverter efficiency for a DC load?
No. For a DC load wired straight to the battery, set efficiency to 100 %. Use the inverter’s figure (often 85 to 95 %) only when the load is on AC through an inverter.
Related
- Ohm’s law and power — the lesson behind this tool.
- Trade math refresher — the lesson behind this tool.
- Ohm’s law & power calculator — watts from volts and amps.
- Efficiency & percent — what the inverter loses.
- Series & parallel resistance
- Reference — the formula sheets and quick tables.
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