Akilowatt-hour(unit symbol:kW⋅horkW h;commonly written askWh) is anon-SIunitofenergyequal to 3.6megajoules(MJ) in SI units, which is the energy delivered by onekilowattof power for onehour.Kilowatt-hours are a common billing unit for electrical energy supplied byelectric utilities.Metric prefixesare used for multiples and submultiples of the basic unit, the watt-hour (3.6 kJ).
| kilowatt-hour | |
|---|---|
 Residentialelectricity meterlocated in Canada | |
| General information | |
| Unit system | Non-SI metric |
| Unit of | Energy |
| Symbol | kW⋅h, kW h |
| Conversions | |
| 1 kW⋅hin... | ... is equal to... |
| SI units | 3.6 MJ |
| CGS units | 3.6×1013erg |
| Imperial units | ≈ 85,429,300ft⋅pdl |
| English Engineering units (US only) | ≈ 2,655,224ft⋅lbf |
Definition
editThe kilowatt-hour is a composite unit of energy equal to one kilowatt (kW) sustained for (multiplied by) one hour. TheInternational System of Units(SI) unit of energy meanwhile is thejoule(symbol J). Because awattis by definition onejoulepersecond,and because there are 3,600secondsin an hour, one kWh equals 3,600kilojoulesor 3.6 MJ.[1][2]
Unit representations
editA widely used representation of the kilowatt-hour iskWh,derived from its component units, kilowatt and hour. It is commonly used in billing for delivered energy to consumers byelectric utilitycompanies, and in commercial, educational, and scientific publications, and in the media.[3][4]It is also the usual unit representation in electrical power engineering.[5]This common representation, however, does not comply with thestyle guideof theInternational System of Units(SI).[6]
Other representations of the unit may be encountered:
- kW⋅handkW hare less commonly used, but they are consistent with the SI. The SI brochure[6]states that in forming a compound unit symbol, "Multiplication must be indicated by a space or ahalf-high (centred) dot(⋅), since otherwise some prefixes could be misinterpreted as a unit symbol. "This is supported by a standard[7]issued jointly by an international (IEEE) and national (ASTM) organization, and by a major style guide.[8]However, the IEEE/ASTM standard allowskWh(but does not mention other multiples of the watt-hour). One guide published byNISTspecifically recommends againstkWh"to avoid possible confusion".[9]
- In 2014, the United States official fuel-economywindow stickerfor electric vehicles used the abbreviationkW-hrs.[10]
- Variations in capitalization are sometimes encountered:KWh,KWH,kwh,etc., which are inconsistent with the International System of Units.
- The notationkW/hfor the kilowatt-hour is incorrect, as it denotes kilowatt per hour.
The hour is a unit of time listed among thenon-SI units acceptedby theInternational Bureau of Weights and Measuresfor use with the SI.[6]
An electric heater consuming 1,000 watts (1 kilowatt) operating for one hour uses one kilowatt-hour of energy. A television consuming 100 watts operating continuously for 10 hours uses one kilowatt-hour. A 40-watt electric appliance operating continuously for 25 hours uses one kilowatt-hour.
Electricity sales
editElectrical energy is typically sold to consumers in kilowatt-hours. The cost of running an electrical device is calculated by multiplying the device's power consumption in kilowatts by the operating time in hours, and by the price per kilowatt-hour. Theunit priceof electricity charged by utility companies may depend on the customer's consumption profile over time. Prices vary considerably by locality. In the United States prices in different states can vary by a factor of three.[11]
While smaller customer loads are usually billed only for energy, transmission services, and the rated capacity, larger consumers 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 changes.[12]Industrial users may also have extra charges according to thepower factorof their load.
Major energy production or consumption is often expressed as terawatt-hours (TWh) for a given period that is often acalendar yearorfinancial year.A 365-day year equals 8,760 hours, so over a period of one year, power of one gigawatt equates to 8.76 terawatt-hours of energy. Conversely, one terawatt-hour is equal to a sustained power of about 114 megawatts for a period of one year.
Examples
editIn 2020, the average household in the United States consumed 893 kWh per month.[13]
Raising the temperature of 1litreof water fromroom temperatureto the boiling point with anelectric kettletakes about 0.1 kWh.
A 12 wattLED lamplit constantly uses about 0.3 kW⋅h per 24 hours and about 9 kWh per month.
In terms ofhuman power,a healthy adult male manual laborer performs work equal to about half a kilowatt-hour over an eight-hour day.[14]
Conversions
editTo 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⋅m2⋅s−2= | 1 | 2.77778 × 10−4 | 2.77778 × 10−7 | 6.241 × 1018 | 0.239 |
| 1 Wh = | 3.6 × 103 | 1 | 0.001 | 2.247 × 1022 | 859.8 |
| 1 kWh = | 3.6 × 106 | 1,000 | 1 | 2.247 × 1025 | 8.598 × 105 |
| 1 eV = | 1.602 × 10−19 | 4.45 × 10−23 | 4.45 × 10−26 | 1 | 3.827 × 10−20 |
| 1 cal = | 4.184 | 1.162 × 10−3 | 1.162 × 10−6 | 2.612 × 1019 | 1 |
Watt-hour multiples
edit| Value | Symbol | Name |
|---|---|---|
| 10−6 | μW⋅h | microwatt-hour |
| 10−3 | mW⋅h | milliwatt-hour |
| 100 | W⋅h | watt-hour |
| 103 | kW⋅h | kilowatt-hour |
| 106 | MW⋅h | megawatt-hour |
| 109 | GW⋅h | gigawatt-hour |
| 1012 | TW⋅h | terawatt-hour |
| 1015 | PW⋅h | petawatt-hour |
All theSI prefixesare commonly applied to the watt-hour: a kilowatt-hour is 1,000 Wh (kWh); a megawatt-hour is 1 million Wh (MWh); a milliwatt-hour is 1/1,000 Wh (mWh) and so on. The kilowatt-hour is commonly used by electrical energy 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(MWh),gigawatt-hours(GWh), andterawatt-hours(TWh) are often used for metering larger amounts of electrical energy to industrial customers and in power generation. The terawatt-hour andpetawatt-hour(PWh) units are large enough to conveniently express theannual electricity generation for whole countriesand theworld energy consumption.
Distinction between kWh (energy) and kW (power)
editA kilowatt is a unit of power (rate of flow of energy per unit of time). A kilowatt-hour is a unit of energy. Kilowatt per hour would be a rate of change of power flow with time.
Workis the amount of energy transferred to a system;poweris therate of deliveryof energy. Energy is measured injoules,orwatt-seconds.Power is measured inwatts,orjoules per second.
For example, a battery stores energy. When the battery delivers its energy, it does so at a certain power, that is, the rate of delivery of the energy. The higher the power, the quicker the battery's stored energy is delivered. A higher power output will cause the battery's stored energy to be depleted in a shorter time period.
Annualized power
editElectric energy production and consumption are sometimes reported on a yearly basis, in units such as megawatt-hours per year (MWh/yr) gigawatt-hours/year (GWh/yr) or terawatt-hours per year (TWh/yr). These units have dimensions of energy divided by time and thus are units of power. They can be converted to SI power units by dividing by the number of hours in a year, about8760 h/yr.
Thus, 1 GWh/yr = 1 GWh/8760 h ≈114.12 kW.
Misuse of watts per hour
editMany compound units for various kinds of rates explicitly mention units of time to indicate a change over time. For example: miles per hour, kilometres per hour, dollars per hour. Power units, such as kW, already measure the rate of energy per unit time (kW=kJ/s). Kilowatt-hours 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 achangeof power per hour, i.e. an acceleration in the delivery of energy. It is used to measure the daily variation of demand (e.g. the slope of theduck curve), or ramp-up behavior ofpower plants.For example, a power plant that reaches a power output of1 MWfrom0 MWin 15 minutes has a ramp-up rate of4 MW/h.
Other uses of terms such aswatts per hourare likely to be errors.[15][16]
Other related energy units
editSeveral other units related to kilowatt-hour are commonly used to indicate power or energy capacity or use in specific application areas.
Average annual energy production or consumption can be expressed in kilowatt-hours per year. This is used with loads or output that vary during the year but whose annual totals are similar from one year to the next. For example, it is useful to compare theenergy efficiencyof household appliances whose power consumption varies with time or the season of the year. Another use is to measure the energy produced by a distributed power source. One kilowatt-hour per year equals about 114.08milliwattsapplied constantly during one year.
The energy content of abatteryis usually expressed indirectly by its capacity inampere-hours;to convert ampere-hour (Ah) to watt-hours (Wh), the ampere-hour value must be multiplied by the voltage of the power source. This value is approximate, since the battery voltage is not constant during its discharge, and because higher discharge rates reduce the total amount of energy that 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 V forLi-ion) that must be used to calculate rather than the device output (for example, usually 5.0 V forUSBportable chargers). This results in a 500 mA USB device running for about 3.7 hours on a 2,500 mAh battery, not five hours.
TheBoard of Trade unit(B.T.U.)[17]is an obsolete UK synonym for kilowatt-hour. The term derives from the name of theBoard of Tradewhich regulated the electricity industry until 1942 when theMinistry of Powertook over.[18]This should not be confused with aBritish Thermal Unit(BTU) which is 1055 J.
In India, the kilowatt-hour is often simply called aunitof energy. A million units, designatedMU,is a gigawatt-hour and aBU(billion units) is a terawatt-hour.[19][20]
See also
editReferences
edit- ^Thompson, Ambler and Taylor, Barry N. (2008).Guide for the Use of the International System of Units (SI)Archived3 June 2016 at theWayback Machine(Special publication 811). Gaithersburg, MD: National Institute of Standards and Technology. 12.
- ^"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.Archived3 June 2016 at theWayback Machine(Special publication 330). Gaithersburg, MD: National Institute of Standards and Technology. 20.
- ^IEC Electropedia, Entry 131-11-58Archived14 March 2016 at theWayback Machine
- ^See for example:Wind Energy Reference Manual Part 2: Energy and Power DefinitionsArchived26 November 2007 at theWayback MachineDanish Wind Energy Association. Retrieved 9 January 2008;"Kilowatt-Hour (kWh)"Archived2 March 2016 at theWayback MachineBusinessDictionary. Retrieved 9 January 2008;"US Nuclear Power Industry"Archived26 November 2007 at theWayback Machineworld-nuclear.org. Retrieved 9 January 2008;"Energy. A Beginners Guide: Making Sense of Units"Archived26 November 2007 at theWayback MachineRenew On Line (UK).TheOpen University.Retrieved 9 January 2008.
- ^ASTM SI10-10, IEEE/ASTM SI 10 American National Standard for Metric Practice, ASTM International, West Conshohocken, PA, 2010, [ 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)"
- ^abc"The International System of Units (SI)"(PDF).International Bureau of Weights and Measures (BIPM). 2019. p. 147.Retrieved7 April2020.
- ^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 EngineersandASTM.15.
- ^"10.57: Units derived from SI base units".The Chicago Manual of Style(17 ed.). Chicago: University of Chicago Press. 2017.Retrieved1 April2020.
- ^"Guide for the Use of the International System of Units (SI)"(PDF).National Institute of Standards and Technology.2008.Retrieved6 April2020.
Reference [4: ISO 31-0] 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 kilowatt-hour. Nevertheless, this Guide takes the position that a half-high dot or a space should always be used to avoid possible confusion;
- ^"Electric Vehicles: Learn More About the New Label".fueleconomy.gov.US Department of energy.Retrieved10 August2014.
- ^Average Price of Electricity to Ultimate Customers by End-Use Sector,U.S. Energy Information Administration, April 2018
- ^"Understanding Electric Demand"Archived6 June 2016 at theWayback Machine,National Grid
- ^"How much electricity does an American home use?".U.S. Energy Information Administration. 7 October 2021.Retrieved11 June2022.
- ^Eugene A. Avallone et al., (ed),Marks' Standard Handbook for Mechanical Engineers 11th Edition,Mc-Graw Hill, New York 2007ISBN0-07-142867-4page 9-4
- ^Woofenden, Ian (5 October 2009).Wind Power For Dummies.John Wiley & Sons. p. 48.ISBN978-0-470-49637-4.
Much too often, I hear people say something like... 'watts per hour'...
- ^Home Power.Electron Connection. 2007.
Watts per hour. This nonsensical phrase tops my electrical terminology pet peeve list.
- ^E. M. Kirkpatrick, ed. (1983).Chambers 20th Century Dictionary(New ed.). Edinburgh: Chambers. p. 137.ISBN0550102345.
- ^"The Board of Trade 1621-1970".Department for Business Innovation and Skills.Archived fromthe originalon 4 March 2010.
- ^"Get enlightened about electricity".The Financial Express.20 December 2004. Archived fromthe originalon 8 September 2012.Retrieved29 November2009.
- ^"BHEL manufactured units generate record power".The Hindu.Press Trust of India.24 July 2008. Archived fromthe originalon 7 November 2012.Retrieved29 November2009.