The watt-hour (symbolized Wh) is a unit of energy equivalent to one watt (1 W) of power expended for one hour (1 h) of time. The watt-hour is not a standard unit in any formal system, but it is commonly used in electrical applications.
An energy expenditure of 1 Wh represents 3600 joules (3.600 x 103 J). To obtain joules when watt-hours are known, multiply by 3.600 x 103. To obtain watt-hours when joules are known, multiply by 2.778 x 10-4.
In general, energy (E) is equivalent to power (P) multiplied by time (t). To determine E in watt-hours, P must be expressed in watts and t must be expressed in hours. Suppose a 60-W bulb burns for 3 h. Then P = 60 and t = 3, so the energy E in watt-hours is:
E = Pt = 60 x 3 = 180 Wh
If P and t are not specified in watts and hours respectively, then they must be converted to those units before determining E in watt-hours.
The watt-hour is rarely used to express energy in any form other than electrical. A quantity of gasoline, oil, or coal contains potential energy that is liberated when the fuel is burned. The heat energy resulting from combustion of such fuels is usually expressed in joules according to the International System of Units (SI) or in British thermal units (Btu) according to the foot-pound-second (fps) or English system. If this energy is used to operate an electric generator, the output of the generator over a certain period of time can be expressed in watt-hours.