The kilowatt hour, or kilowatthour, (symbol kW·h, kW h or kWh) is a unit of energy equal to 1000 watthours or 3.6 megajoules.^{[1]}^{[2]}
For constant power, energy in watthours is the product of power in watts and time in hours. The kilowatthour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.
Definition
The kilowatthour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour.
 $\backslash mathrm\{kWh\}=(3600\backslash ,\backslash mathrm\{s\})(\backslash mathrm\{kW\})=3600\backslash ,\backslash mathrm\{s\}\backslash Bigg(\backslash frac\{\backslash mathrm\{kJ\}\}\{\backslash mathrm\{s\}\}\backslash Bigg)=3600\backslash ,\backslash mathrm\{kJ\}$
Inversely, one watt is equal to 1 J/s. One kilowatthour 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 kilowatthour (equivalent to 3.6 megajoules) of energy. A 60watt light bulb consumes 0.06 kilowatthours of energy per hour. Electrical energy is sold in kilowatthours; cost of running equipment is the product of power in kilowatts multiplied by running time and price per kilowatthour. The unit price of electricity may depend upon the rate of consumption and the time of day.
Symbol and abbreviation for kilowatt hour
The international standard for SI^{[3]} states that in forming a compound unit symbol, "Multiplication must be indicated by a space or a halfhigh (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^{[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
Template:See
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

kilowatt hour

electronvolt

calorie

1 J = 1 kg·m^{2} s^{−2} =

1

2.77778 × 10^{−4}

2.77778 × 10^{−7}

6.241 × 10^{18}

0.239

1 W·h =

3,600

1

0.001

2.247 × 10^{22}

859.8

1 kW·h =

3.6 × 10^{6}

1,000

1

2.247 × 10^{25}

8.598 × 10^{5}

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 × 10^{19}

1

Watt hour multiples and billing units
The kilowatthour 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 kilowatthours. Megawatthours, gigawatthours, and terawatthours are often used for metering larger amounts of electrical energy to industrial customers and in power generation. The terawatthour and petawatthour are large enough to conveniently express annual electricity generation for whole countries.
SI multiples for watt hour (W·h)
Submultiples 

Multiples 
Value 
Symbol 
Name 

Value 
Symbol 
Name 
10^{−3}  mW·h  milliwatt hour   10^{3}  kW·h  kilowatt hour 
10^{−6}  µW·h  microwatt hour   10^{6}  MW·h  megawatt hour 
    10^{9}  GW·h  gigawatt hour 
    10^{12}  TW·h  terawatt hour 
    10^{15}  PW·h  petawatt hour 
In India, the kilowatthour is often simply called a Unit of energy. A million units, designated MU, is a gigawatthour and a BU (billion units) is a terawatthour.^{[8]}^{[9]}
Other energyrelated units
Several other units are commonly used to indicate power or energy capacity or use in specific application areas.
All the SI prefixes may be applied to the watthour: a kilowatthour is 1,000 W·h (symbols kW·h, kWh or kW h; a megawatthour is 1 million W·h, (symbols MW·h, MWh or MW h); a milliwatthour is 1/1000 W·h (symbols mW·h, mWh or mW h) and so on.
Average annual power production or consumption can be expressed in kilowatthours 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 kilowatthour per year equals about 114.08 milliwatts applied constantly during one year.
The energy content of a battery is usually expressed indirectly by its capacity in amperehours; to convert watthours (W·h) to amperehour (A·h), the watthour 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 (BOTU) is an obsolete UK synonym for kilowatthour. The term derives from the name of the Board of Trade which regulated the electricity industry until 1942 when the Ministry of Power took over.^{[10]} 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 BTU.
Burnup of nuclear fuel is normally quoted in megawattdays per tonne (MW·d/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.
Confusion of kilowatthours (energy) and kilowatts (power)
The terms power and energy are frequently confused. Physical power can be defined as work per unit time, measured in units of joules per second or watts. To produce power over any given period of time requires energy. Either higher levels of power (for a given period) or longer periods of run time (at a given power level) require more energy.
An electrical load (e.g. a lamp, toaster, electric motor, etc.) has a rated "size" in watts. This is its running power level, which equates to the instantaneous rate at which energy must be generated and consumed to run the device. How much energy is consumed at that rate depends on how long you run the device. However, its power level requirements are basically constant while running. The unit of energy for residential electrical billing, kilowatthours, integrates changing power levels in use at the residence over the past billing period (nominally 720 hours for a 30day month), thus showing cumulative electrical energy use for the month.
For another example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt hours (W·h), 0.1 kilowatthour, or 360 kJ. This same amount of energy would light a 40watt bulb for 2.5 hours, or a 10watt lowenergy bulb for 10 hours. A power station electricity output at any particular moment would be measured in multiples of watts, but its annual energy sales would be in multiples of watthours. A kilowatthour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 MJ.
Major energy production or consumption is often expressed as terawatthours (TWh) for a given period that is often a calendar year or financial year. One terawatthour is equal to a sustained power of approximately 114 megawatts for a period of one year.
Misuse of watts per hour
Power units measure the rate of energy per unit time. Many compound units for rates explicitly mention units of time, for example, miles per hour, kilometers per hour, dollars per hour. Kilowatthours are a product of power and time, not a rate of change of power with time.
Watts per hour (W/h) is a unit of a change of power per hour. It might be used to characterize the rampup behavior of power plants. For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a rampup rate of 4 MW/h. Hydroelectric power plants have a very high rampup rate, which makes them particularly useful in peak load and emergency situations.
The proper use of terms such as watts per hour is uncommon, whereas misuse^{[11]} may be widespread.
See also
References
External links
 University of Illinois at UrbanaChampaign has developed an applet which illustrates the consumption and cost of energy in the home, and allows the user to see the effects of manipulating the flow of electricity to various household appliances.
 Prices per kilowatt hour in the USA, Energy Information Administration
This article was sourced from Creative Commons AttributionShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, EGovernment Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a nonprofit organization.