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.
Related resources
12V battery runtime examples
100Ah 12V battery
A 100Ah 12V battery is roughly 1,200 Wh before usable-capacity limits. At an 85% usable setting, planning energy is closer to 1,020 Wh.
100W load
A 100W load on 1,020 usable Wh is about 10.2 hours before other real-world limitations. The calculator does this math from your inputs.
Fridge or pump
A cycling load may not draw its rated watts continuously, but startup surge and inverter limits still matter.
Use this with the power calculators hub
Battery runtime, portable power station runtime, generator sizing, and extension cord voltage drop are linked problems. Use the Power Calculators Hub when you are planning a complete backup or portable power setup.