Kilowatt hour
The kilowatthour (symbol kWh,^{[1]} kW⋅h or kW h^{[2]}) is a derived unit of energy equal to 3.6 megajoules.^{[3]}^{[4]} If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatthours is the power in kilowatts multiplied by the time in hours. The kilowatthour is commonly used as a billing unit for energy delivered to consumers by electric utilities.
Contents
Definition
The kilowatthour (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 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.
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 kilowatthour a nonSI unit of energy. The kilowatthour is not listed among the nonSI units accepted by the BIPM for use with the SI, although the hour, from which the kilowatthour is derived, is.^{[5]}
Examples
An electric heater rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatthour (equivalent to 3.6 megajoules) of energy. A television rated at 100 watts operating for 10 hours continuously uses one kilowatthour. A 40watt light bulb operating continuously for 25 hours uses one kilowatthour.
Electrical energy is sold in kilowatthours; cost of running equipment is the product of power in kilowatts multiplied by running time in hours and price per kilowatthour. 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.
Symbol and abbreviations for kilowatthour
The symbol "kWh" is commonly used in commercial, educational, scientific and media publications,^{[6]} and is the usual practice in electrical power engineering.^{[7]}
Other abbreviations and symbols may be encountered:
 "kW h" is less commonly used. It is consistent with SI standards (but note that the kilowatthour is a nonSI unit). The international standard for SI^{[5]} 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^{[8]} issued jointly by an international (IEEE) and national (ASTM) organization. However, at least one major usage guide^{[9]} and the IEEE/ASTM standard allow "kWh" (but do not mention other multiples of the watthour). One guide published by NIST specifically recommends avoiding "kWh" "to avoid possible confusion".^{[10]}
 "kW·h" is, like "kW h", preferred with SI standards, but it is very rarely used in practice.
 The US official fueleconomy window sticker for electric vehicles uses the abbreviation "kWhrs".^{[11]}
 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 kilowatthour, is not correct.
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  watthour  kilowatthour  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 =  3.6 × 10^{3}  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 
Watthour multiples and billing units
All the SI prefixes are commonly 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. 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 (MWh), gigawatthours (GWh), and terawatthours (TWh) are often used for metering larger amounts of electrical energy to industrial customers and in power generation. The terawatthour and petawatthour (PWh) units are large enough to conveniently express the annual electricity generation for whole countries.
Submultiples  Multiples  

Value  Symbol  Name  Value  Symbol  Name  
10^{−3}  mW·h  milliwatthour  10^{3}  kW·h  kilowatthour  
10^{−6}  µW·h  microwatthour  10^{6}  MW·h  megawatthour  
10^{9}  GW·h  gigawatthour  
10^{12}  TW·h  terawatthour  
10^{15}  PW·h  petawatthour 
Confusion of kilowatthours (energy) and kilowatts (power)
The terms power and energy are frequently confused. Power is defined as the rate of delivery of energy, the work performed per unit time, measured in units joules per second or watts. To produce power over any 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, TVset, electric motor, 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. The amount of energy that is consumed at that rate depends on how long the device is operated. However, its power requirements are 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, which is 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 100 watts is turned on for one hour, the energy used is 100 watthours (W·h), 0.1 kilowatthour, or 360 kilojoules. 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 megajoules.
Energy cost can be illustrated in a similar example: a 100watt (i.e. 0.1 kilowatt) light bulb turned on for 8 hours uses 800 W⋅h or 0.8 kW⋅h. If the energy cost is 25 cents per kilowatthour, it would cost 0.1 kW × 8 h × $0.25/kW⋅h = $0.20.
Whereas individual homes only pay for the kilowatthours consumed, commercial buildings and institutions also pay for peak power consumption, the greatest power recorded in a fairly short time, such as 15 minutes. This compensates the power company for maintaining the infrastructure needed to provide peak power. These charges are billed as demand charges.^{[12]}
Major energy production or consumption is often expressed as terawatthours (TW⋅h) 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^{[13]} may be widespread.
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 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 ampere hour (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, and because higher discharge rates reduce the total amount of energy the battery can provide. In the case of devices that output a different voltage than the battery, it is the battery voltage (typically 3.7 for Liion) that must be used to calculate rather than the device output (usually 5.0 for USB portable chargers), since use of the lower figure allows manufacturers of such devices to overstate the capacity or runtime, inflating it by 35% ( ^{5}⁄_{3.7}=1.35). This results in a 500mA USB device running for only about 3.7 hours on a 2500mAh battery, rather than the expected five hours.
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.^{[14]} 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.
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.^{[15]}^{[16]}
Burnup of nuclear fuel is normally quoted in megawatt days 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.^{[citation needed]}
See also
References
 ^ IEC Electropedia, Entry 1311158 Archived March 14, 2016, at the Wayback Machine.

^ "Guide for the Use of the International System of Units (SI)" (PDF). physics.nist.gov. National Institute of Standards and Technology. 2008. Archived (PDF) from the original on 3 June 2016. Retrieved 25 January 2017.
Reference [4: ISO 310] suggests that if a space is used to indicate units formed by multiplication, the space may be omitted if it does not cause confusion. This possibility is reflected in the common practice of using the symbol kWh rather than kW · h or kW h for the kilowatthour. Nevertheless, this Guide takes the position that a halfhigh dot or a space should always be used to avoid possible confusion;
 ^ Thompson, Ambler and Taylor, Barry N. (2008). Guide for the Use of the International System of Units (SI) Archived June 3, 2016, at the Wayback Machine. (Special publication 811). Gaithersburg, MD: National Institute of Standards and Technology. 12.
 ^ "Halfhigh 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. Archived June 3, 2016, at the Wayback Machine. (Special publication 330). Gaithersburg, MD: National Institute of Standards and Technology. 20.
 ^ ^{a} ^{b} The International System of Units (SI) Archived April 29, 2016, at the Wayback Machine.. (2006, 8th ed.) Paris: International Bureau of Weights and Measures. 130.
 ^ See for example: Wind Energy Reference Manual Part 2: Energy and Power Definitions Archived November 26, 2007, at the Wayback Machine. Danish Wind Energy Association. Retrieved 9 January 2008; "KilowattHour (kWh)" Archived March 2, 2016, at the Wayback Machine. BusinessDictionary.com. Retrieved 9 January 2008; "US Nuclear Power Industry" Archived November 26, 2007, at the Wayback Machine. www.worldnuclear.org. Retrieved 9 January 2008; "Energy. A Beginners Guide: Making Sense of Units" Archived November 26, 2007, at the Wayback Machine. Renew On Line (UK). The Open University. Retrieved 9 January 2008.
 ^ ASTM SI1010, IEEE/ASTM SI 10 American National Standard for Metric Practice, ASTM International, West Conshohocken, PA, 2010, [www.astm.org] "The symbols for certain compound units of electrical power engineering are usually written without separation, thus: watthour (Wh), kilowatthour (kWh), voltampere (VA), and kilovoltampere (kVA)"
 ^ Standard for the Use of the International System of Units (SI): The Modern Metric System. (1997). (IEEE/ASTM SI 101997). New York and West Conshohocken, PA: Institute of Electrical and Electronics Engineers and ASTM. 15.
 ^ Chicago Manual of Style. (14th ed., 1993) University of Chicago Press. 482.
 ^ Guide for the Use of the International System of Units (SI) p.12 Archived March 4, 2016, at the Wayback Machine.
 ^ "Electric Vehicles: Learn More About the New Label". fueleconomy.gov. US Department of energy. Retrieved 10 August 2014.
 ^ " Understanding Electric Demand" Archived June 6, 2016, at the Wayback Machine., National Grid
 ^ "Inverter Selection". Northern Arizona Wind and Sun. Retrieved 27 March 2009.
 ^ "The Board of Trade 16211970". Archived from the original on 2010.
 ^ "Get enlightened about electricity". The Financial Express. December 20, 2004. Archived from the original on September 8, 2012. Retrieved 29 November 2009.
 ^ "BHEL manufactured units generate record power". The Hindu. Press Trust of India. July 24, 2008. Archived from the original on November 7, 2012. Retrieved 29 November 2009.
External links
 Prices per kilowatthour in the USA, Energy Information Administration