- Definition
- Watt-hour rating is the total energy a battery can store, calculated as its capacity multiplied by its nominal voltage, and it is the figure that decides which lithium battery transport rules apply.
This page defines the term. For what it means in practice, see Power bank capacity claims: what a mAh rating actually means.
The calculation
Watt-hours are the capacity in amp-hours multiplied by the nominal voltage.
The reason this figure matters more than milliamp-hours is that milliamp-hours are voltage dependent and watt-hours are not. Two batteries with the same milliamp-hour rating at different voltages hold different amounts of energy, so comparing them in milliamp-hours is meaningless.
Why it decides things
Transport rules for lithium batteries are written in watt-hours rather than milliamp-hours. The threshold a battery falls above or below determines whether it can travel as a standalone shipment, whether it must be carried as cargo rather than in the cabin, how it must be packed and labelled, and what documentation the shipper must hold.
Where it appears
The watt-hour figure should appear on the product marking, in the transport documentation, and in the test summary required for the battery. Where a product is described only in milliamp-hours, the watt-hour figure can be derived from the nominal voltage — but the derived figure is not a substitute for the one the manufacturer is required to declare.