Power capacity is how much energy is stored in the battery. This power is often expressed in Watt-hours (the symbol Wh). A Watt-hour is the voltage (V) that the battery provides multiplied by how much current (Amps) the battery can provide for some amount of time (generally in hours). Voltage * Amps * hours = Wh.
Capacity ratings only tell you how much energy the cell can store and provide. They don’t give you any information about the power of the cell or its longevity. The highest capacity batteries usually have only moderate power levels. There is often a tradeoff between power rating and capacity.
Since this is a particularly confusing part of measuring batteries, I'm going to discuss it more in detail. Power capacity is how much energy is stored in the battery. This power is often expressed in Watt-hours (the symbol Wh).
The capacity of lithium battery cells is measured in amp-hours (Ah) or sometimes milliamp-hours (mAh) where 1 Ah = 1,000 mAh. Lithium battery cells can have anywhere from a few mAh to 100 Ah. Occasionally the unit watt-hour (Wh) will be listed on a cell instead of the amp-hour. Watt-hour is another unit of energy, but also consider voltage.
The standard operating voltage of a battery is indicated by a reference value known as nominal voltage. It is a standardized measurement that illustrates the voltage range in which a battery typically functions.
The capacity of a cell is probably the most critical factor, as it determines how much energy is available in the cell. The capacity of lithium battery cells is measured in amp-hours (Ah) or sometimes milliamp-hours (mAh) where 1 Ah = 1,000 mAh. Lithium battery cells can have anywhere from a few mAh to 100 Ah.