mAh vs Wh: milliamp-hours need voltage before they describe energy
Milliamp-hours describe electric charge capacity. Watt-hours describe energy. You can convert from mAh to Wh only when you also know the voltage associated with that capacity rating.
Convert mAh and volts to watt-hours
mAh to Wh calculator
This is a unit conversion using the voltage you enter. It does not predict usable runtime, conversion losses, battery health, or the voltage profile during discharge.
mAh is a charge quantity, not an energy quantity
The ampere is the SI unit of electric current. When current is accumulated over time, ampere-hours describe electric charge. NIST's SI conversion tables give one ampere-hour as exactly 3,600 coulombs, so one milliamp-hour is one-thousandth of that charge quantity.
That makes mAh useful when comparing batteries under a shared electrical context, but the number does not contain voltage. Two batteries can both be labeled 2,000 mAh and still represent different amounts of energy if their rated voltages differ.
Wh includes the voltage term
Electrical power is voltage multiplied by current. Extend that relationship across time and the unit relationship becomes voltage × ampere-hours = watt-hours.
NIST's conversion tables also give one watt-hour as exactly 3,600 joules. Watt-hours are therefore an energy quantity: they describe how much energy corresponds to the stated capacity and voltage, not how quickly that energy is being used.
Example: the same mAh number at different voltages
| Example rating | Calculation | Energy |
|---|---|---|
| 2,000 mAh at 1.2 V | 2,000 × 1.2 ÷ 1,000 | 2.4 Wh |
| 2,000 mAh at 3.7 V | 2,000 × 3.7 ÷ 1,000 | 7.4 Wh |
| 2,000 mAh at 7.4 V | 2,000 × 7.4 ÷ 1,000 | 14.8 Wh |
The charge capacity is identical in all three rows, but the energy is not. That is why “this battery has more mAh” is not a complete energy comparison unless the relevant voltages are also comparable.
Why power-bank labels can be confusing
A power bank may contain internal cells with one nominal voltage while delivering power through a USB output at a different voltage. Conversion electronics change the voltage and incur losses. The mAh figure printed for the internal cell capacity therefore should not be multiplied by an unrelated output voltage as if both numbers described the same point in the system.
If a manufacturer provides a watt-hour rating, that energy figure is usually the cleaner starting point for comparing total stored energy. If you calculate Wh yourself, use the voltage that belongs to the stated Ah or mAh capacity rating rather than a convenient USB output voltage from somewhere else on the product.
Why a 10,000 mAh label does not mean 10 Ah at every voltage
Suppose a hypothetical battery stores 37 Wh and its cell-capacity rating is stated as 10 Ah at 3.7 V. If ideal conversion changed that energy to 5 V, conservation of energy means the available ampere-hours at the new voltage could not still be 10 Ah: 37 Wh ÷ 5 V equals 7.4 Ah before conversion losses.
Real conversion is not lossless, so the actual delivered charge at the output would be lower still under the stated conditions. This is another reason to use watt-hours when the goal is to compare energy across systems operating at different voltages.
mAh, Wh, and W answer three different questions
| Unit | What it describes | What it does not tell you by itself |
|---|---|---|
| mAh | Electric charge capacity | Total energy without the associated voltage |
| Wh | Energy | How fast that energy is being delivered or consumed |
| W | Power, an energy-use or transfer rate | How much total energy is stored without a time/energy quantity |
For the relationship between power and energy over time, see watts versus watt-hours. For USB charging negotiation and charger limits, see USB-C charger wattage.
Runtime still requires more than Wh
If a battery stores 60 Wh and a device draws a constant 30 W, simple arithmetic gives two ideal hours. That does not guarantee two real hours. Conversion losses, battery-management limits, temperature, cell aging, varying device load, cutoff behavior, and the fact that battery voltage changes during discharge can all affect usable runtime.
Use Wh ÷ W as a transparent first-order estimate, then treat product-specific runtime claims as measurements that need their own test conditions.
A reliable comparison workflow
- Write down the capacity number and its exact unit.
- If the capacity is in mAh or Ah, identify the voltage associated with that rating.
- Convert to Wh before comparing stored energy across different voltages.
- Do not mix an internal-cell mAh rating with an unrelated output voltage.
- Keep stored energy (Wh) separate from power rate (W).
- For real runtime, include conversion efficiency and the device's changing power draw rather than treating the ideal quotient as a guarantee.
Primary sources
- NIST Guide to the SI, Appendix B.9, which lists 1 A·h = 3,600 C and energy conversion factors.
- NIST Guide to the SI, Appendix B.8, which lists 1 W·h = 3,600 J.
- NIST — SI Units: Electric Current, for the ampere and the relationship 1 V = 1 W/A.
Bottom line
mAh tells you charge capacity. Wh tells you energy. To move from one to the other, voltage is not optional: divide mAh by 1,000 to get Ah, then multiply by the voltage associated with that capacity rating.