Watt-Hours (Wh) Calculator

What is a watt-hour?

A watt-hour (Wh) is a unit of energy equal to one watt of power sustained for one hour. Battery makers usually print a capacity in milliamp-hours (mAh), which measures charge rather than energy. Two batteries with the same mAh rating can store very different amounts of energy if their voltages differ, so mAh alone does not tell you how much work a battery can do.

This calculator converts a capacity in mAh and a nominal voltage in volts (V) into the stored energy in watt-hours and kilowatt-hours (kWh). It is handy for comparing power banks, sizing devices, and — because airlines limit carry-on batteries by watt-hours — checking whether a battery is allowed on a flight.

How does the calculator work?

Energy is the product of charge and voltage. A capacity in milliamp-hours is a charge in milliamp-hours; multiplying by the voltage gives milliwatt-hours, and dividing by 1000 converts to watt-hours. Dividing once more by 1000 gives kilowatt-hours.

Enter the capacity and the voltage, and the calculator returns both the energy in Wh and the same value in kWh.

Formula

$E_{\text{Wh}} = \frac{C_{\text{mAh}} \times V}{1000}$

$E_{\text{kWh}} = \frac{E_{\text{Wh}}}{1000}$

where $C_{\text{mAh}}$ is the capacity in milliamp-hours and $V$ is the voltage in volts.

Worked examples

Example 1 — a phone battery. A 3000 mAh cell at 3.7 V:

$E_{\text{Wh}} = \frac{3000 \times 3.7}{1000} = 11.1\ \text{Wh}$

That is 0.0111 kWh — comfortably under the 100 Wh limit most airlines allow without special approval.

Example 2 — a small power station. A 5000 mAh pack at 12 V:

$E_{\text{Wh}} = \frac{5000 \times 12}{1000} = 60\ \text{Wh}$

Conversion reference

Notes and practical uses

• Use the battery’s nominal voltage (for a single lithium-ion cell this is about 3.7 V), not its fully charged voltage, for a realistic energy figure.
• To go the other way — from watt-hours back to milliamp-hours — see the mAh to Wh calculator.
• For power rather than energy, the watt calculator relates voltage, current, and resistance.

FAQ

Why can’t I compare batteries by mAh alone? Because mAh measures charge, not energy. A 10000 mAh cell at 3.7 V stores 37 Wh, while the same 10000 mAh at 12 V stores 120 Wh — over three times the energy.

What voltage should I use? Use the nominal voltage of the battery or pack. For multi-cell packs, that is the sum of the cell voltages.

How does this relate to airline battery limits? Airlines rate carry-on batteries in watt-hours. Most allow up to 100 Wh freely and 100–160 Wh with airline approval, so converting mAh to Wh tells you which category a battery falls into.