Amp-hours to watt-hours converter (Ah ↔ Wh)
Type a number into either box. Enter amp-hours to get watt-hours, or watt-hours to get amp-hours. The voltage is what links the two, so pick your battery type or enter its nominal voltage.
The same energy at other voltages
| Battery type | Nominal voltage | Amp-hours needed |
|---|
The formula
Ah = Wh ÷ V
Ah = mAh ÷ 1,000
An amp-hour measures charge: one amp flowing for one hour. A watt-hour measures energy. To go from one to the other you need the voltage, because the same amount of charge carries four times as much energy at 48 V as it does at 12 V. That's the reason you can't compare two batteries by amp-hours unless they share a voltage.
Use the nominal voltage printed on the battery, not the voltage you measure. A 12 V LiFePO4 battery reads anywhere from about 10 V to 14.6 V depending on how full it is and whether it's charging. Its nominal 12.8 V is the average used for capacity ratings.
Worked examples
| Battery | Calculation | Result |
|---|---|---|
| 100 Ah LiFePO4, 12.8 V | 100 × 12.8 | 1,280 Wh |
| 100 Ah lead-acid, 12 V | 100 × 12 | 1,200 Wh rated, about 600 Wh if you only use half |
| 20,000 mAh power bank, 3.7 V cells | 20,000 ÷ 1,000 × 3.7 | 74 Wh |
| Power station listed as 1,024 Wh, 12.8 V pack | 1,024 ÷ 12.8 | 80 Ah |
| 5,120 Wh home battery, 51.2 V | 5,120 ÷ 51.2 | 100 Ah |
The first two rows show why amp-hours alone can mislead. A 100 Ah lead-acid battery and a 100 Ah LiFePO4 battery look identical on paper. But lead-acid batteries last much longer if you regularly use only about half their capacity, so in practice the lithium one gives you roughly twice the usable energy. The "capacity you plan to use" setting lets you account for that.
Why your power bank doesn't deliver its full mAh
Power banks list their capacity in mAh at the voltage of the lithium cells inside, usually 3.6 V or 3.7 V. Your phone charges over USB at 5 V or higher. So a 20,000 mAh bank holds about 74 Wh, and at 5 V that's at most about 14,800 mAh (74 ÷ 5 × 1,000). Conversion losses in the bank's circuitry take a bit more. The bank isn't lying. The mAh figure just refers to a different voltage than the one your phone sees.
Watt-hour limits for flying
US airline rules for lithium-ion batteries are written in watt-hours, which is a common reason to need this conversion. According to the FAA's PackSafe guidance:
- 100 Wh or less: generally allowed in carry-on bags.
- 101–160 Wh: allowed only with airline approval, with a limit of two spare batteries per person.
- Over 160 Wh: not allowed in passenger baggage. That rules out nearly every portable power station.
- Spare batteries and power banks must go in carry-on, not checked bags.
Airlines can set stricter limits than the FAA, and rules change. Check the FAA's lithium battery page and your airline before you travel. If the battery's label shows a Wh rating, use that number rather than a converted one.
Frequently asked questions
How many watt-hours is a 100 Ah battery?
It depends on the voltage. At 12 V it's 1,200 Wh, at 12.8 V (LiFePO4) it's 1,280 Wh, at 24 V it's 2,400 Wh and at 48 V it's 4,800 Wh.
Does a bigger Ah rating mean longer runtime?
Only between batteries with the same voltage. Convert to watt-hours first, then use the battery runtime calculator to turn that into hours for a specific device.
Why do lead-acid batteries list a "20-hour rate"?
A lead-acid battery's amp-hour rating usually assumes it's discharged slowly, over about 20 hours. Drain it faster, as an inverter running a microwave would, and it delivers noticeably less. Lithium batteries hold their rated capacity much better at high currents. Check the datasheet if your loads are heavy.
Which chemistry is my battery?
The label or datasheet usually says. The nominal voltage is a good clue too: 12.8 V, 25.6 V and 51.2 V point to LiFePO4, while 12 V and 24 V are typical of lead-acid. For more on the two common lithium types, read LiFePO4 vs. NMC batteries.