Watt-hours measure energy, which is what you need to compare batteries. Amp-hours alone do not tell you that, because a battery’s energy also depends on its voltage. Multiply amp-hours by voltage to get watt-hours.
- Watt-hours
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The formula
Watt-hours (Wh) = amp-hours (Ah) × voltage (V).
| Battery | Calculation | Energy |
|---|---|---|
| 100 Ah at 12 V | 100 × 12 | 1,200 Wh |
| 100 Ah at 12.8 V (typical LiFePO4 nominal) | 100 × 12.8 | 1,280 Wh |
| 50 Ah at 51.2 V | 50 × 51.2 | 2,560 Wh |
| 20,000 mAh at 3.7 V (power bank cells) | 20 × 3.7 | 74 Wh |
Why the voltage matters
Two batteries with the same amp-hour rating can hold very different amounts of energy if their voltages differ. Power bank capacities in mAh are usually quoted at the internal cell voltage (around 3.6 to 3.7 V), so less energy reaches a phone that charges at 5 V or more after conversion losses.
Remember that the rated watt-hours are not all usable in practice; see the battery runtime calculator and watt-hours vs amp-hours for the details.
