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Kilowatt-hour (kWh) is a unit used to measure electrical energy expended or used over time. It is commonly used as a unit of electrical energy in engineering, academic and commercial applications. One kilowatt-hour is the equivalent of one kilowatt of power transferred or consumed in an hour. Generally the symbol “kWh” is used in general media publications, however "kW h" is its preferred symbol since it is consistent with SI standards.
A kilowatt-hour is equivalent to 3.6 × 106 J (3.6 megajoules) of energy. It is equivalent to one kilowatt of power that is sustained for one hour. It is basically power (P) in kilowatts (kW) multiplied by time (t) in hours (h):
E (kW h) = P (kW) t (h)
The international standard unit for energy is the joule and the hour is not a SI unit for time, so kilowatt-hour is not a SI standard unit of energy. It is a product of power and time and should not be confused with kilowatts per hour (kW/h), which is a unit for measuring the rate of change of power over time, such as when describing the ramp-up behavior of power plants. For example, a power generator that can reach a power output of 1 megawatt from 0 in 15 minutes has a ramp-up rate of 4 megawatts per hour (MW/h).
The kilowatt-hour is used by electrical distribution providers for purposes of billing of households and other customers that do not consume large amounts of power. But for metering bigger quantities of electrical energy to industrial customers and for referring to power generation, megawatt-hours (MWh), gigawatt-hours (GWh) and terawatt-hours (TWh) are used.
To give an idea what a kilowatt-hour of electricity is equivalent to, consider the following examples: