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"KWH" redirects here. For other uses, see KWH (disambiguation).

The kilowatt hour, also written kilowatt-hour,1 (symbol kW·h, kW h or kWh) is a unit of energy.2

Energy delivered by electric utilities is usually expressed and charged for in kWh.

Note that the kWh is the product of power in kilowatts multiplied by time in hours; it is not kW/h.

Contents

Definition

The SI (International System of Units) unit of energy is the joule (J), equal to one watt - second; one kilowatt hour is exactly 3.6 megajoules, which is the amount of energy transferred if work is done at a rate of one thousand watts for one hour.

Other energy related units

The kilowatt hour is a convenient unit for electrical bills because the energy usage of a typical electrical customer in one month is several hundred kilowatt hours. Megawatt hours and terawatt hours are used for metering larger amounts of electrical energy.

The energy delivered by batteries is usually expressed indirectly in ampere-hours; to convert watt-hour (Wh) into ampere-hour (Ah), the watt-hour value must be multiplied by the voltage of the power source.3

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.


The Board of Trade unit or 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.

Burnup of nuclear fuel is normally quoted in megawatt days per ton (MWd/MTU), where ton 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.

Examples

If a heater is rated at 1000 watts (1 kilowatt) and that heater is on for one hour then one kilowatt hour is used.

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 that is 30 days X 100/1000 = 3 kilowatt hours.

4

Multiples

SI multiples for watt hour (W·h)
Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 W·h dW·h deciwatt hour 101 W·h daW·h decawatt hour
10–2 W·h cW·h centiwatt hour 102 W·h hW·h hectowatt hour
10–3 W·h mW·h milliwatt hour 103 W·h kW·h kilowatt hour
10–6 W·h µW·h microwatt hour 106 W·h MW·h megawatt hour
10–9 W·h nW·h nanowatt hour 109 W·h GW·h gigawatt hour
10–12 W·h pW·h picowatt hour 1012 W·h TW·h terawatt hour
10–15 W·h fW·h femtowatt hour 1015 W·h PW·h petawatt hour
10–18 W·h aW·h attowatt hour 1018 W·h EW·h exawatt hour
10–21 W·h zW·h zeptowatt hour 1021 W·h ZW·h zettawatt hour
10–24 W·h yW·h yoctowatt hour 1024 W·h YW·h yottawatt hour
Common multiples are in bold face


Symbol and abbreviation for kilowatt hour

The brochure for SI5 and a voluntary standard6 issued jointly by an international (IEEE) and national (ASTM) organization state 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"). However, at least one major usage guide7 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".8 Nonetheless, it is commonly used in commercial, educational, scientific and media publications.9

Conversions

from / to joule watt hour electronvolt calorie
1 J = 1 kg m2 s-2 = 1 0.278 × 10−3 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

See also

Energy portal

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. ^ "Ah (ampere-hour) to Wh (watt-hour) conversion" BatteryHippo.com, 2008, webpage: BatteryHippo.com.
  4. ^ Frequently Asked Questions. (2008). QueryCAT. Retrieved 20 November 2008.
  5. ^ The International System of Units (SI). (2006, 8th ed.) Paris: International Bureau of Weights and Measures. 130.
  6. ^ 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.
  7. ^ Chicago Manual of Style. (14th ed., 1993) University of Chicago Press. 482.
  8. ^ Taylor, Barry N. (1995). 13
  9. ^ 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.

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