What is the difference between an Amp-Hour and a Watt-Hour?

All cells and batteries, being electrochemical sources of electrical potential, can only maintain that electrical potential for as long as the chemicals that create that potential exist inside the battery. As soon as the cell or battery is connected into the device it is designed to power, current immediately begins to flow though the device, and as a result, the chemicals inside the cell or battery which create that electrical potential are consumed. Therefore how much chemical material exists in the cell or battery must be declared so device designers and users know how long the device will be powered by the cell or battery. This declaration is known as a cell or battery’s capacity or energy.

Capacity is always represented in terms of Amp-Hours (Ah) and energy in terms of Watt-Hours (Wh). The practical understanding of those units is dependent on your method of discharge. Typically, safety and comparative characterization testing of a cell or battery is preferred to be discharged using constant current, where amperage is the primary unit of measure. Devices, which typically have a constant power requirement, use wattage as the primary unit of measurement. Therefore, capacity and energy ratings can quickly tell a user how long a cell or battery will last when being discharged by either known unit of measure.

Watt-Hours (Wh) is a equivalent to the multiplication of a cell or battery’s voltage by its Amp-Hour (Ah) rating.