Why you need the voltage
A kilowatt-hour is an amount of energy. An amp-hour is an amount of charge. They are not the same kind of quantity, and there is no fixed conversion between them — it depends entirely on the voltage the charge sits at. A 1.2 kWh battery is 100 Ah at 12 V, 50 Ah at 24 V, or 25 Ah at 48 V. Feed the calculator the energy and the system voltage and it returns the charge.
This comes up when a battery or storage system is specified in kWh — as home batteries and EV packs usually are — but you need amp-hours to size a charge controller, set a battery monitor, or pick cable.
The kWh to amp-hours formula
Multiply kilowatt-hours by 1,000 to get watt-hours, then divide by the nominal system voltage. The calculator also shows the intermediate watt-hour figure.
Worked example
The calculator’s defaults are 1.2 kWh at 12 V.
Watt-hours:
Amp-hours:
So 1.2 kWh at 12 V is 100 Ah of nominal capacity.
kWh to amp-hours at common voltages
| Energy | 12 V | 24 V | 48 V |
|---|---|---|---|
| 1.2 kWh | 100 Ah | 50 Ah | 25 Ah |
| 2.4 kWh | 200 Ah | 100 Ah | 50 Ah |
| 5 kWh | 417 Ah | 208 Ah | 104 Ah |
| 10 kWh | 833 Ah | 417 Ah | 208 Ah |
The amp-hour figures at 12 V get large fast. A 10 kWh bank at 12 V is 833 Ah — a lot of parallel cells and very heavy interconnects. The same 10 kWh at 48 V is 208 Ah, which is why nearly every home-scale battery system runs at 48 V or higher.
Nominal versus usable
The result is nameplate capacity. Usable amp-hours are fewer:
At 80% depth of discharge and 90% system efficiency, the 100 Ah example gives
Common mistakes
- Using the charging voltage instead of nominal, which understates the amp-hours.
- Sizing a 12 V bank for a large kWh target and ending up with impractical cable and cell counts.
- Treating nominal Ah as usable Ah.
For the reverse, use the amp-hours to kWh calculator. For runtime under a load, see the battery runtime calculator.