Kilowatt-hour: Wikis

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Residential electric power meter located in United States.

The kilowatt hour, or kilowatt-hour, (symbol kW·h, kW h) is a unit of energy equal to 1000 watt hours or 3.6 megajoules.[1][2]

Energy in watt hours is the multiplication of power in watts and time in hours.

The kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.

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. 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.

Examples

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.

Symbol and abbreviation for kilowatt hour

The international standard for SI[3] 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[4] issued jointly by an international (IEEE) and national (ASTM) organization. However, at least one major usage guide[5] 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".[6] Nonetheless, it is commonly used in commercial, educational, scientific and media publications.[7]

Conversions

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.

joule watt hour electronvolt calorie
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

Watt hour multiples and billing units

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.

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

In India, the kilowatt hour is often simply called a Unit of energy. A million units, designated MU, is a gigawatt hour and a BU (billion units) is a terawatt hour.[8][9]

Other energy-related units

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.[10]

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.

References

1. ^ Taylor, Barry N. (1995). Guide for the Use of the International System of Units (SI) (Special publication 811). Gaithersburg, MD: National Institute of Standards and Technology. 31.
2. ^ "Half-high dots or spaces are used to express a derived unit formed from two or more other units by multiplication." Barry N. Taylor. (2001 ed.) The International System of Units. (Special publication 330). Gaithersburg, MD: National Institute of Standards and Technology. 20
3. ^ The International System of Units (SI). (2006, 8th ed.) Paris: International Bureau of Weights and Measures. 130.
4. ^ Standard for the Use of the International System of Units (SI): The Modern Metric System. (1997). (IEEE/ASTM SI 10-1997). New York and West Conshohocken, PA: Institute of Electrical and Electronics Engineers and ASTM. 15.
5. ^ Chicago Manual of Style. (14th ed., 1993) University of Chicago Press. 482.
6. ^ Taylor, Barry N. (1995). 13
7. ^ See for example: Wind Energy Reference Manual Part 2: Energy and Power Definitions Danish Wind Energy Association. Retrieved 9 January 2008; "Kilowatt-Hour (kWh)" BusinessDictionary.com. Retrieved 9 January 2008; "US Nuclear Power Industry" www.world-nuclear.org. Retrieved 9 January 2008; "Energy. A Beginners Guide: Making Sense of Units" Renew On Line (UK). The Open University. Retrieved 9 January 2008.
8. ^ "Get enlightened about electricity". The Financial Express. December 20, 2004. Retrieved 29 November 2009.
9. ^ "BHEL manufactured units generate record power". The Hindu. PTI. July 24, 2008. Retrieved 29 November 2009.
10. ^ "Ah (ampere-hour) to Wh (watt-hour) conversion" BatteryHippo.com, 2008, webpage: BatteryHippo.com.