• Kilowatt hour

    Kilowatt hour

    • The kilowatt-hour (symbol kWh,kW·h or kW h) is a derived unit of energy equal to 3.6 megajoules. If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt-hours is the power in kilowatts multiplied by the time in hours. The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.

      The kilowatt-hour (symbolized kW⋅h as per SI) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour.

      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.

      The base unit of energy within the International System of Units (SI) is the joule. The hour is a unit of time "outside the SI", making the kilowatt-hour a non-SI unit of energy. The kilowatt-hour is not listed among the non-SI units accepted by the BIPM for use with the SI, although the hour, from which the kilowatt-hour is derived, is.

      An electric heater rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt-hour (equivalent to 3.6 megajoules) of energy. A television rated at 100 watts operating for 10 hours continuously uses one kilowatt-hour. A 40-watt light bulb operating continuously for 25 hours uses one kilowatt-hour.

      Electrical energy is sold in kilowatt-hours; cost of running equipment is the product of power in kilowatts multiplied by running time in hours and price per kilowatt-hour. The unit price of electricity may depend upon the rate of consumption and the time of day. Industrial users may also have extra charges according to their peak usage and the power factor.

      The symbol "kWh" is commonly used in commercial, educational, scientific and media publications, and is the usual practice in electrical power engineering.

      joule watt-hour kilowatt-hour electronvolt calorie
      1 J = 1 kg·m2 s−2 = 1 2.77778 × 10−4 2.77778 × 10−7 6.241 × 1018 0.239
      1 W·h = 3.6 × 103 1 0.001 2.247 × 1022 859.8
      1 kW·h = 3.6 × 106 1,000 1 2.247 × 1025 8.598 × 105
      1 eV = 1.602 × 10−19 4.45 × 10−23 4.45 × 10−26 1 3.827 × 10−20
      1 cal = 4.1868 1.163 × 10−3 1.163 × 10−6 2.613 × 1019 1
      SI multiples for watt-hour (W·h)
      Submultiples Multiples
      Value Symbol Name Value Symbol Name
      10−3 mW·h milliwatt-hour 103 kW·h kilowatt-hour
      10−6 µW·h microwatt-hour 106 MW·h megawatt-hour
      109 GW·h gigawatt-hour
      1012 TW·h terawatt-hour
      1015 PW·h petawatt-hour

      • "kW h" is less commonly used. It is consistent with SI standards (but note that the kilowatt-hour is a non-SI unit). The international standard for SI states that in forming a compound unit symbol, "Multiplication must be indicated by a space or a half-high (centered) 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".
      • "kW·h" is, like "kW h", preferred with SI standards, but it is very rarely used in practice.
      • The US official fuel-economy window sticker for electric vehicles uses the abbreviation "kW-hrs".
      • Variations in capitalization are sometimes seen: KWh, KWH, kwh, etc. these are inconsistent with International System of Units.
      • The notation "kW/h", as a symbol for kilowatt-hour, is not correct.
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    • Kilowatt hour