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Kilowatt hour

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The kilowatt hour (symbol kWh or kW·h) is a unit of energy equal to 1000 watt hours or 3.6 megajoules. Energy in kilowatt hours is the multiplication of power in kilowatts and time in hours. The kilowatt hour is most commonly known as a billing unit for electrical energy delivered to consumers by electric service providers.

Definition

The standard unit of energy in the International System of Units (SI) is the joule (J), equal to one watt second. Inversely, one watt is equal to 1 J/s and thus one kilowatt hour is 3.6 × 106 joules or 3.6 megajoules, which is the amount of energy expended if work is done at a constant rate of one thousand watts for one hour.

Examples

• A heater, rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt hour (equivalent to 3.6 megajoules) of energy.
• Using a 60 watt light bulb for one hour consumes 0.06 kilowatt hours of energy in the form 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 Wh = 3 kWh (equivalent to 10.8 megajoules).

Symbol for kilowatt hour

The International System of Units[1] states that when compound unit symbols are formed by multiplication, the individual symbols should be separated by a half-high dot or a space (for example, "kW·h" or "kW h"). One guide published by NIST[2] specifically recommends avoiding "kWh" in order "to avoid possible confusion". However, "kWh" continues to be very commonly used in commercial, educational, scientific and media publications.

References

1. Bureau International des Poids et Mesures (2006). The International System of Units (SI). 8th edition. See Section 5.1 Symbols.
2. Taylor, Barry N. (1995). Guide for the Use of the International System of Units (SI) Special publication 811, National Institute of Standards and Technology. See pages 12 – 13 of the guide (pages 24 – 25 of the PDF).