When you need this conversion
Power banks, phone batteries, and small solar chargers are almost always rated in mAh, but comparing true capacity, checking airline carry-on limits, or matching a power bank to a small solar panel's output all require watt-hours (Wh). mAh alone is meaningless for comparison unless the voltage is the same.
The formula in plain terms
Watt-hours equal milliamp-hours multiplied by voltage, divided by 1000 to convert milliamps to amps: Wh = mAh times V divided by 1000. Most single-cell lithium-ion and lithium-polymer batteries use a nominal voltage of 3.7 V, so a device or power bank rated in mAh can be converted to Wh directly using that figure unless the datasheet states a different nominal voltage.
Common mAh to Wh values (at 3.7 V)
| mAh | Wh |
|---|---|
| 3,000 mAh | 11.1 Wh |
| 5,000 mAh | 18.5 Wh |
| 10,000 mAh | 37.0 Wh |
| 20,000 mAh | 74.0 Wh |
| 27,000 mAh | 99.9 Wh |
Practical tips
Watch the 100 Wh airline threshold
Most airlines cap carry-on power banks at 100 Wh without approval, which is why many large power banks are marketed just under 27,000 mAh: at 3.7 V that lands just under the 100 Wh line.
Check the cell chemistry before assuming 3.7 V
Some devices use LiFePO4 cells (3.2 V nominal) or multi-cell packs at higher nominal voltages. Using the wrong voltage in the formula will give an incorrect Wh figure, so check the label or datasheet.
Expect losses from internal conversion
A power bank's rated Wh is the raw cell capacity, not the energy delivered through the USB port. Voltage step-up circuitry and charging losses typically reduce real-world output by 10-20 percent.