The kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.
Inversely, one watt is equal to 1 J/s. One kilowatt hour is 3.6 megajoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.
A heater, rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt hour (equivalent to 3,600 kilojoules) of energy.
Using a 60 watt light bulb for one hour consumes 0.06 kilowatt hours of electricity. Using a 60 watt light bulb for one thousand hours consumes 60 kilowatt hours of electricity.
If a 100 watt light bulb is on for one hour per day for 30 days, the energy used is 100 W × 30 h = 3000 W·h = 3 kW·h, the equivalent of 10.8 million joules.
The international standard for SI states that in a forming a compound unit symbol, "Multiplication must be indicated by a space or a half-high (centred) dot (·), since otherwise some prefixes could be misinterpreted as a unit symbol" (i.e., "kW h" or "kW·h"). This is supported by a voluntary standard issued jointly by an international (IEEE) and national (ASTM) organization. However, at least one major usage guide and the IEEE/ASTM standard allow "kWh" (but do not mention other multiples of the watt hour). One guide published by NIST specifically recommends avoiding "kWh" "to avoid possible confusion". Nonetheless, it is commonly used in commercial, educational, scientific and media publications.
To convert a quantity measured in a unit in the left column to the units in the top row, multiply by the factor in the cell where the row and column intersect.
|1 J = 1 kg m2 s-2 =||1||2.778 × 10−4||6.241 × 1018||0.239|
|1 W·h =||3600||1||2.247 × 1022||859.8|
|1 eV =||1.602 × 10−19||4.45 × 10−23||1||3.827 × 10−20|
|1 cal =||4.1868||1.163 × 10−3||2.613 × 1019||1|
The kilowatt hour is commonly used by electrical distribution providers for purposes of billing, since the monthly energy consumption of a typical residential customer ranges from a few hundred to a few thousand kilowatt hours. Megawatt hours, gigawatt hours, and terawatt hours are often used for metering larger amounts of electrical energy to industrial customers and in power generation.
|10-3||mW·h||milliwatt hour||103||kW·h||kilowatt hour|
|10-6||µW·h||microwatt hour||106||MW·h||megawatt hour|
Several other units are commonly used to indicate power or energy capacity or use in specific application areas.
Average annual power production or consumption can be expressed in kilowatt hours per year; for example, when comparing the energy efficiency of household appliances whose power consumption varies with time or the season of the year, or the energy produced by a distributed power source. One kilowatt hour per year equals about 114.08 milliwatts applied constantly during one year.
The energy capacity of a battery is usually expressed indirectly in ampere hours; to convert watt hours (W·h) to ampere hour (A·h), the watt hour value must be divided by the voltage of the power source. This value is approximate since the voltage is not constant during discharge of a battery.
The Board of Trade unit (B.O.T.U.) is an obsolete UK synonym for kilowatt hour. The term derives from the name of the Board of Trade that regulated the electricity industry. The B.O.T.U. should not be confused with the British thermal unit or BTU, which is a much smaller quantity of thermal energy. To further the confusion, at least as late as 1937, Board of Trade unit was simply abbreviated "B.T.U." or "BTU".
Burnup of nuclear fuel is normally quoted in megawatt-days per tonne (MWd/MTU), where tonne refers to a metric ton of uranium metal or its equivalent, and megawatt refers to the entire thermal output, not the fraction which is converted to electricity.