Battery runtime makes more sense when you use watt-hours.
Runtime depends on usable battery capacity and the watts your device actually draws. Amp-hours can be useful, but only when you also know voltage. Watt-hours make it easier to compare batteries and power stations.
Watt-hours vs amp-hours
Amp-hours do not tell the full story unless voltage is known. A 100Ah battery at 12V is very different from 100Ah at a lower voltage.
- Watt-hours roughly equal volts × amp-hours.
- Compare watt-hours when battery voltages are different.
- Use usable capacity, not just the label rating.
- Inverters and battery management systems reduce real runtime.
Loads are not always steady
Many devices do not draw the same wattage all the time.
- Fridges, pumps, compressors, and heaters may cycle on and off.
- Motors can have startup surge higher than running watts.
- Inverters have maximum continuous and surge ratings.
- Cold weather and battery age can reduce usable energy.
How to use the estimate
Use the calculator as a planning check, then verify the battery and device ratings before relying on it.
- Enter battery watt-hours if known.
- Enter average device watts or nameplate watts.
- Use an efficiency factor for inverter and usable-capacity loss.
- Oversize for critical loads.
Common questions
How do I calculate 12V battery runtime?
Convert amp-hours to watt-hours by multiplying by voltage, then divide usable watt-hours by the load in watts. Real runtime can be lower due to inverter loss, cutoff settings, temperature, and battery age.
Why use watt-hours instead of amp-hours?
Watt-hours include voltage, which makes different batteries easier to compare. Amp-hours alone can be misleading if voltages differ.