Watthour

Watthour, commonly written watt-hour, symbol Wh, is a unit of energy equal to the amount of energy transferred when one watt of power is sustained for one hour. It is widely used to express electrical energy consumption or storage.

Power is the rate at which energy is transferred, measured in watts (W). Energy equals power multiplied by time, so energy in watt-hours is P (W) × t (h). Therefore, 1 Wh equals 1 W sustained for 1 hour. In joules, 1 Wh equals 3600 joules. The watt-hour is not an SI base unit; the SI unit of energy is the joule, but the watt-hour is a widely used non-SI unit accepted for use with SI in electrical contexts.

The kilowatt-hour (kWh) is the standard billing unit for electricity. 1 kWh equals 1000 Wh, or 3.6 million joules. Larger units include the megawatt-hour (MWh) and gigawatt-hour (GWh). Conversely, smaller practical measures include watt-hours for individual devices and short-term energy estimates.

Examples help illustrate usage: a 60 W light bulb running for 1 hour consumes 60 Wh (0.06 kWh). If it runs for 24 hours, it uses 1,440 Wh or 1.44 kWh. Battery capacities are often expressed in Wh or kWh; the stored energy approximates voltage times capacity in ampere-hours (Wh = V × Ah).

Summary: the watthour is a convenient unit for quantifying electrical energy, defined as power times time, with 1 Wh = 3600 J and 1 kWh = 1000 Wh, commonly used in energy accounting and battery specifications.