Watt

{{Short description|SI derived unit of power}}
{{About|the unit of power}}
{{redirect|MWT|the former Australian theatre company|Melbourne Workers Theatre}}
{{hatnote|"Joules per second" redirects here, it should not be confused with [[Joule-second]]}}
{{Infobox Unit
| bgcolour =
| name = watt
| image = 
| caption =
| standard = [[SI]]
| quantity = [[Power (physics)|power]]
| symbol = W
| namedafter = [[James Watt]]
| units1 = [[SI base unit]]s
| inunits1 = 1 [[kilogram|kg]]&sdot;[[metre|m]]<sup>2</sup>&sdot;[[second|s]]<sup>−3</sup>
| units2 = [[CGS unit]]s
| inunits2 = {{val|e=7|u=[[erg]]⋅[[second|s]]<sup>−1</sup>}}
| units3 = [[English Engineering Units]]
| inunits3 = {{cvt|1|W|ftlbf/s|sigfig=7|disp=out}} = {{cvt|1|W|hp|sigfig=7|disp=out}}
}}

The '''watt''' (symbol: '''W''') is the unit of [[Power (physics)|power]] or [[radiant flux]] in the [[International System of Units|International System of Units (SI)]], equal to 1 [[joule]] per [[second]] or 1&nbsp;kg⋅m<sup>2</sup>⋅s<sup>−3</sup>.<ref>{{cite report | last1=Newell | first1=David B | last2=Tiesinga | first2=Eite | title=The international system of units (SI) | publisher=National Institute of Standards and Technology | publication-place=Gaithersburg, MD | year=2019 | doi=10.6028/nist.sp.330-2019 | url = https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf}}  §2.3.4, Table 4.</ref><ref>{{Cite book|last1=Yildiz|first1=I.|title=Comprehensive energy systems. Vol 1: Energy fundamentals|last2=Liu|first2=Y.|publisher=Elsevier|year=2018|isbn=9780128149256|editor1-last=Dincer|editor1-first=I.|pages=12–13|chapter=Energy units, conversions, and dimensional analysis}}</ref><ref>{{SIBrochure8th|pages=118, 144}}</ref> It is used to [[quantification (science)|quantify]] the rate of [[Work (physics)|energy transfer]]. The watt is named in honor of [[James Watt]] (1736–1819), an 18th-century [[Scottish people|Scottish]] [[invention|inventor]], [[mechanical engineer]], and [[chemist]] who improved the [[Newcomen steam engine|Newcomen engine]] with his own [[Watt steam engine|steam engine]] in 1776. Watt's invention was fundamental for the [[Industrial Revolution]].

==Overview==
When an object's [[velocity]] is held constant at one [[meter per second]] against a constant opposing force of one [[Newton (unit)|newton]], the rate at which [[Work (physics)|work]] is done is one watt.
<math display=block>\mathrm{1 ~ W = 1 ~ J {/} s = 1 ~ N {\cdot} m {/} s = 1 ~ kg {\cdot} m^2 {\cdot} s^{-3}}.</math>

In terms of [[electromagnetism]], one watt is the rate at which [[electrical work]] is performed when a current of one [[ampere]] (A) flows across an electrical [[potential difference]] of one [[volt]] (V), meaning the watt is equivalent to the [[volt-ampere]] (the latter unit, however, is used for a different quantity from the [[real power]] of an electrical circuit).
<math display=block>\mathrm{1 ~ W = 1 ~ V \times 1 ~ A}.</math>

Two additional [[Conversion of units|unit conversions]] for watt can be found using the above equation and [[Ohm's law]].
<math display=block>\mathrm{1 ~ W = 1 ~ V^2 / \Omega = 1 ~ A^2 {\cdot} \Omega},</math>
where [[ohm]] (<math>\Omega</math>) is the [[SI derived unit]] of [[electrical resistance]].

=== Examples ===
*A person having a mass of 100&nbsp;kg who climbs a 3-meter-high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to [[gravity]] times height divided by the time it takes to lift the object to the given height gives the ''rate of doing work'' or ''power''.{{efn-lr|The energy in climbing the stairs is given by {{mvar|mgh}}. Setting {{math|''m'' {{=}} 100 kg}}, {{math|''g'' {{=}} 9.8 m/s<sup>2</sup>}} and {{math|''h'' {{=}} 3 m}} gives 2940&nbsp;J.  Dividing this by the time taken (5&nbsp;s) gives a power of 588&nbsp;W. }}
* A laborer over the course of an eight-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.<ref>{{Citation | editor1-first = Eugene A | editor1-last = Avallone |display-editors=etal | year = 2007 | title = Marks' Standard Handbook for Mechanical Engineers | edition = 11th | publisher = Mc-Graw Hill | place = New York | isbn = 978-0-07-142867-5 | pages = 9–4}}.</ref>

==Origin and adoption as an SI unit==
The watt is named after the Scottish inventor [[James Watt]].<ref name=Klein>{{cite book |last=Klein |first=Herbert Arthur |year=1988 |location=New York |publisher=Dover |orig-year=1974 |title=The Science of measurement: A historical survey |isbn=9780486144979 |page=239 }}</ref> The unit name was proposed by [[Carl Wilhelm Siemens|C. William Siemens]] in August 1882 in his President's Address to the Fifty-Second Congress of the [[British Science Association|British Association for the Advancement of Science]].<ref name=Siemens>{{cite encyclopedia |url=https://www.biodiversitylibrary.org/item/95237#page/85/mode/1up |title= Address by C. William Siemens|pages=1–33|encyclopedia= Report of the Fifty-Second meeting of the British Association for the Advancement of Science |location=London| publisher = John Murray | year = 1883 |volume= 52}}</ref> Noting that units in the [[CGS#Practical cgs units|practical system of units]] were named after leading physicists, Siemens proposed that ''watt'' might be an appropriate name for a unit of power.<ref>Siemens supported his proposal by asserting that  Watt was the first who "had a clear physical conception of power, and gave a rational method for measuring it." [https://www.biodiversitylibrary.org/item/95237#page/90/mode/1up "Siemens, 1883, p. 6"]</ref> Siemens defined the unit within the existing system of practical units as "the power conveyed by a current of an [[Ampere|Ampère]] through the difference of potential of a Volt".<ref>{{Cite web|url=https://www.biodiversitylibrary.org/item/95237|title=Report of the British Association for the Advancement of Science|date=April 3, 1883|volume=52nd Meeting (1882)}}</ref>

In October 1908, at the International Conference on Electric Units and Standards in London,<ref>{{cite book | author=Tunbridge, P. | title=Lord Kelvin: His Influence on Electrical Measurements and Units | location=Peter Peregrinus | publisher=London | year=1992 |page=51 | isbn=0-86341-237-8}}</ref> so-called ''international'' definitions were established for practical electrical units.<ref name=EB11-742>{{cite EB1911|wstitle= Units, Physical | volume= 27 | pages = 738&ndash;745; see page 742 |last= Fleming |first= John Ambrose |author-link= John Ambrose Fleming}}</ref> Siemens' definition was adopted as the ''international'' watt. (Also used: {{nowrap|1 A<sup>2</sup> ×}} 1&nbsp;Ω.)<ref name=Klein/> The watt was defined as equal to 10<sup>7</sup> units of power in the ''practical system'' of units.<ref name=EB11-742/> The [[International System of Electrical and Magnetic Units#Overdefinition and the 1908 modification|"international units"]] were dominant from 1909 until 1948. After the 9th [[General Conference on Weights and Measures]] in 1948, the ''international'' watt was redefined from practical units to absolute units (i.e., using only length, mass, and time). Concretely, this meant that 1 watt was defined as the quantity of energy transferred in a unit of time, namely 1&nbsp;J/s. In this new definition, 1 ''absolute'' watt&nbsp;= 1.00019 ''international'' watts. Texts written before 1948 are likely to be using the ''international'' watt, which implies caution when comparing numerical values from this period with the post-1948 watt.<ref name=Klein/> In 1960, the 11th General Conference on Weights and Measures adopted the ''absolute'' watt into the [[International System of Units]] (SI) as the unit of power.<ref>{{cite web |url=https://www.bipm.org/en/CGPM/db/11/12/ |title= Resolution 12 of the 11th CGPM (1960)|author=<!--Not stated--> |publisher=  Bureau International des Poids et Mesures (BIPM) |access-date=9 April 2018 |archive-date=April 20, 2020 |archive-url=https://web.archive.org/web/20200420104247/https://www.bipm.org/en/CGPM/db/11/12/}}</ref> 
<!--{{SI unit lowercase|James Watt|watt|W}}-->

== Multiples ==
{{For|additional examples of magnitude for multiples and submultiples of the watt|Orders of magnitude (power)}}
{{SI multiples
|unit=watt
|symbol=W
|note=Common multiples are in '''bold''' face
|p=|n=|mc=|m=|k=|M=|G=|T=|P=|f=
}}

;<span id="Attowatt">Attowatt</span>: The sound intensity in water corresponding to the international standard reference [[sound pressure]] of 1&nbsp;[[micropascal|μPa]] is approximately 0.65&nbsp;aW/m<sup>2</sup>.<ref>Ainslie, M. A. (2015). A century of sonar: Planetary oceanography, underwater noise monitoring, and the terminology of underwater sound. Acoustics Today.</ref>
;<span id="Femtowatt">Femtowatt</span>: Powers measured in femtowatts are typically found in references to [[radio]] and [[radar]] receivers. For example, meaningful [[FM tuner]] performance figures for sensitivity, quieting and [[Signal-to-noise ratio|signal-to-noise]] require that the [[Radio frequency|RF]] energy applied to the antenna input be specified. These input levels are often stated in dBf ([[decibel]]s referenced to 1 femtowatt). This is 0.2739 microvolts across a 75-ohm load or 0.5477 microvolt across a 300-ohm load; the specification takes into account the RF [[input impedance]] of the tuner.
;<span id="Picowatt">Picowatt</span>: Powers measured in picowatts are typically used in reference to radio and radar receivers, [[acoustics]] and in the science of [[radio astronomy]]. One picowatt is the international standard reference value of [[sound power]] when this quantity is expressed in decibels.<ref>Morfey, C.L. (2001). Dictionary of Acoustics.</ref>
;<span id="Nanowatt">Nanowatt</span>: Powers measured in nanowatts are also typically used in reference to radio and radar receivers.
;<span id="Microwatt">Microwatt</span>: Powers measured in microwatts are typically stated in [[medical instrument]]ation systems such as the [[Electroencephalography|electroencephalograph]] (EEG) and the [[Electrocardiography|electrocardiograph]] (ECG), in a wide variety of scientific and engineering instruments and also in reference to radio and radar receivers. Compact [[solar cells]] for devices such as [[Solar-powered calculator|calculators]] and [[Solar-powered watch|watches]] are typically measured in microwatts.<ref>{{Citation | newspaper = The New York Times | url = https://www.nytimes.com/2010/07/18/business/18novel.html | title = Bye-Bye Batteries: Radio Waves as a Low-Power Source | date = Jul 18, 2010 | url-status = live | archive-url = https://web.archive.org/web/20170321231716/http://www.nytimes.com/2010/07/18/business/18novel.html | archive-date = 2017-03-21 }}.</ref>
;<span id="Milliwatt">Milliwatt</span>: A typical [[laser pointer]] outputs about five milliwatts of light power, whereas a typical [[hearing aid]] uses less than one milliwatt.<ref>{{cite web | url = http://www.datasheetarchive.com/datasheet-pdf/019/DSA00333218.html | title = Low-Power Real-Time Programmable DSP Development Platform for Digital Hearing Aids | first1 = Trudy | last1 = Stetzler | first2 = Neeraj | last2 = Magotra | first3 = Pedro | last3 = Gelabert | first4 = Preethi | last4 = Kasthuri | first5 = Sridevi | last5 = Bangalore | publisher = Datasheet Archive | access-date = 8 February 2010 | url-status = live | archive-url = https://web.archive.org/web/20110303094710/http://www.datasheetarchive.com/datasheet-pdf/019/DSA00333218.html | archive-date = 3 March 2011 }}</ref> [[Audio signal]]s and other electronic signal levels are often measured in [[dBm]], referenced to one milliwatt.
;<span id="Kilowatt">Kilowatt</span>
{{Redirect2 |Kilowatt|Kilowatts|the musician James Watts|KiloWatts (musician)}}
{{redirect|kW}}
:The kilowatt is typically used to express the output power of [[engine]]s and the power of [[electric motor]]s, tools, machines, and heaters. It is also a common unit used to express the [[Electromagnetic radiation|electromagnetic]] power output of broadcast radio and television [[transmitter]]s. {{paragraph}} One kilowatt is approximately equal to 1.34 [[horsepower]]. A small electric heater with one [[heating element]] can use 1 kilowatt. The average [[Electric energy consumption|electric power consumption]] of a household in the United States is about 1 kilowatt.{{efn-lr|Average household electric power consumption is 1.19&nbsp;kW in the US, 0.53&nbsp;kW in the UK.  In India it is 0.13&nbsp;kW (urban) and 0.03&nbsp;kW (rural) – computed from GJ figures quoted by Nakagami, Murakoshi and Iwafune.<ref>{{cite conference
 |conference    = ACEEE Summer Study on Energy Efficiency in Buildings
 |year          = 2008
 |conference-url = http://aceee.org/conferences/2008/ssb
 |publisher     = American Council for an Energy-Efficient Economy
 |location      = [[Pacific Grove, California]]
 |title         = International Comparison of Household Energy Consumption and Its Indicator
 |url           = http://www.aceee.org/files/proceedings/2008/data/papers/8_24.pdf
 |first1        = Hidetoshi
 |last1         = Nakagami
 |first2        = Chiharu
 |last2         = Murakoshi
 |first3        = Yumiko
 |last3         = Iwafune
 |at            = Figure 3. Energy Consumption per Household by Fuel Type. 8:214–8:224
 |access-date    = 14 February 2013
 |url-status       = live
 |archive-url    = https://web.archive.org/web/20150109012214/http://www.aceee.org/files/proceedings/2008/data/papers/8_24.pdf
 |archive-date   = 9 January 2015
}}</ref>
}} {{paragraph}} A surface area of 1 square meter on Earth receives typically about one kilowatt of sunlight from the Sun (the [[solar irradiance]]) (on a clear day at midday, close to the equator).<ref>Elena Papadopoulou, ''Photovoltaic Industrial Systems: An Environmental Approach'', Springer 2011 {{ISBN|3642163017}},  p.153</ref>
;<span id="Megawatt">Megawatt</span>: Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; large [[server farm]]s or [[Data center#Energy use|data centers]]; and some scientific research equipment, such as [[supercollider]]s, and the output pulses of very large lasers. A large residential or commercial building may use several megawatts in electric power and heat. On railways, modern high-powered [[electric locomotive]]s typically have a peak power output of {{val|5|or|6|u=MW}}, while some produce much more. The [[British Rail Class 373|Eurostar e300]], for example, uses more than {{val|12|u=MW}}, while heavy [[Diesel electric locomotive|diesel-electric locomotives]] typically produce and use {{val|3|and|5|u=MW}}. U.S. [[nuclear power plant]]s have net summer capacities between about {{val|500|and|1300|u=MW}}.<ref>{{cite report | chapter-url = https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1350/v19/sr1350v19.pdf | publisher = [[Nuclear Regulatory Commission|United States Nuclear Regulatory Commission]] | language = en-us | date = 2007-08-01 | title = 2007–2008 Information Digest | chapter = Appendix A {{!}} U.S. Commercial Nuclear Power Reactors | pages = 84{{hyphen}}101 | issue = NUREG-1350 | volume = 19 | access-date = 2021-12-27 | url-status = dead | archive-url = https://web.archive.org/web/20080216073347/http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1350/v19/sr1350v19.pdf | archive-date = 2008-02-16 | df = dmy-all }}</ref>{{rp|pp=84{{hyphen}}101}} {{paragraph}} The earliest citing of the megawatt in the ''[[Oxford English Dictionary]]'' (''OED'') is a reference in the 1900 [[Webster's Dictionary|''Webster's International Dictionary of the English Language'']]. The ''OED'' also states that ''megawatt'' appeared in a 28 November 1947 article in the journal ''[[Science (journal)|Science]]'' (506:2).
[[File:Office of Nuclear Energy video explaining gigawatts.ogg|thumb|A [[United States Department of Energy]] video explaining gigawatts]]
;<span id="Gigawatt">Gigawatt</span>: A gigawatt is typical average power for an industrial city of one million habitants and also the output of a large power station. The GW unit is thus used for large power plants and [[Electrical grid|power grids]]. For example, by the end of 2010, power shortages in China's Shanxi province were expected to increase to 5–6&nbsp;GW<ref>{{cite web | place = Peking | first1 = Jim | last1 = Bai | first2 = Aizhu | last2 = Chen | editor-first = Chris | editor-last = Lewis | url = http://in.reuters.com/article/idINTOE6AA0AD20101111 | title = China's Shanxi to face 5–6 GW power shortage by yr-end – paper | date = 11 November 2010 | publisher = Reuters}}</ref> and the installation capacity of wind power in Germany was 25.8 GW.<ref>{{cite news | url=http://www.economist.com/node/16846774 | title=Not on my beach, please | newspaper=The Economist | date=19 August 2010 | url-status=live | archive-url=https://web.archive.org/web/20100824080835/http://www.economist.com/node/16846774 | archive-date=24 August 2010 }}</ref> The largest unit (out of four) of the Belgian [[Doel Nuclear Power Station]] has a peak output of 1.04&nbsp;GW.<ref>{{cite web | language = fr | series = Who are we: Nuclear | url= http://www.electrabel.com/whoarewe/nuclear/keyfigures_doel.aspx | title = Chiffres clés |trans-title=Key numbers | year = 2011 | work = Electrabel |archive-url=https://web.archive.org/web/20110710180653/http://www.electrabel.com/whoarewe/nuclear/keyfigures_doel.aspx |archive-date=2011-07-10}}</ref> [[HVDC converter]]s have been built with power ratings of up to 2&nbsp;GW.<ref>{{Citation | last1 = Davidson | first1 = CC | last2 = Preedy | first2 = RM | last3 = Cao | first3 = J | last4 = Zhou | first4 = C | last5 = Fu | first5 = J | contribution = Ultra-High-Power Thyristor Valves for HVDC in Developing Countries | publisher = [[Institution of Engineering and Technology|IET]] | title = 9th International Conference on AC/DC Power Transmission | place = London | date = October 2010}}.</ref>
;<span id="Terawatt">Terawatt</span>: The [[primary energy]] used by humans worldwide was about 160,000&nbsp;terawatt-hours in 2019, corresponding to an average continuous power consumption of 18&nbsp;TW that year.<ref>{{Cite journal |url=https://ourworldindata.org/grapher/global-primary-energy?country=~OWID_WRL |title=Global Direct Primary Energy Consumption |author1=Hannah Ritchie |author1-link=Hannah Ritchie |author2=Max Roser |author2-link=Max Roser |journal=Our World in Data |publisher=Published online at OurWorldInData.org. |year=2020 |access-date=2020-02-09}}</ref> The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for [[nanosecond]] intervals. The average lightning strike peaks at 1&nbsp;TW, but these strikes only last for 30 [[microsecond]]s.
;<span id="Petawatt">Petawatt</span>: A petawatt can be produced by the current generation of lasers for time scales on the order of picoseconds. One such laser is [[Lawrence Livermore National Laboratory|Lawrence Livermore]]'s [[Nova (laser)|Nova laser]], which achieved a power output of 1.25&nbsp;PW by a process called [[chirped pulse amplification]]. The duration of the pulse was roughly 0.5&nbsp;[[picosecond|ps]], giving a total energy of 600&nbsp;J.<ref>{{cite web |url = https://www.llnl.gov/str/Petawatt.html |title = Crossing the Petawatt threshold |publisher = Lawrence Livermore National Laboratory |location = [[Livermore, California|Livermore]], [[California|CA]] |access-date = 19 June 2012 |url-status = live |archive-url = https://web.archive.org/web/20120915212555/https://www.llnl.gov/str/Petawatt.html |archive-date = 15 September 2012 }}</ref> Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE), [[Osaka University]], which achieved a power output of 2&nbsp;PW for a duration of approximately 1&nbsp;[[picosecond|ps]].<ref>{{citation | title = World's most powerful laser: 2 000 trillion watts. What's it? | date = 12 August 2015 | publisher = IFL Science | url = http://www.iflscience.com/technology/world-s-most-powerful-laser-2000-trillion-watts-what-s-it | url-status = live | archive-url = https://web.archive.org/web/20150822093000/http://www.iflscience.com/technology/world-s-most-powerful-laser-2000-trillion-watts-what-s-it | archive-date = 2015-08-22 }}.</ref><ref>{{Citation | title = Eureka alert | type = publicity release | date = Aug 2015 | url = http://www.eurekalert.org/pub_releases/2015-08/ou-wpl080615.php | url-status = live | archive-url = https://web.archive.org/web/20150808055653/http://www.eurekalert.org/pub_releases/2015-08/ou-wpl080615.php | archive-date = 2015-08-08 }}.</ref> {{paragraph}} Based on the average total solar irradiance of 1.361&nbsp;kW/m<sup>2</sup>,<ref name=TSI>{{cite web | title = Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present | publisher = PMODWRC | url = http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant | place = [[Switzerland|CH]] | access-date = 2005-10-05 | url-status = live | archive-url = https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant | archive-date = 2011-08-30 }}</ref> the total power of sunlight striking Earth's atmosphere is estimated at 174&nbsp;PW. The planet's average rate of global warming, measured as [[Earth%27s_energy_budget#Earth's energy imbalance (EEI)|Earth's energy imbalance]], reached about 0.5&nbsp;PW (0.3% of incident solar power) by 2019.<ref>{{cite journal |last1=Loeb |first1=Norman G. |last2=Johnson |first2=Gregory C. |last3=Thorsen |first3=Tyler J. |last4=Lyman |first4=John M. |last5=Rose |first5=Fred G. |last6=Kato |first6=Seiji |display-authors=4 |title=Satellite and Ocean Data Reveal Marked Increase in Earth's Heating Rate |journal=Geophysical Research Letters |date=15 June 2021 |volume=48 |issue=13 |doi=10.1029/2021GL093047 |bibcode=2021GeoRL..4893047L |doi-access=free }}</ref>
;<span id="Yottawatt">Yottawatt</span>: The power output of the Sun is 382.8&nbsp;YW, about 2 billion times the power estimated to reach Earth's atmosphere.<ref name="Sun Fact Sheet">{{cite web |last1=Williams |first1=David R. |title=Sun Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html |website=nasa.gov |publisher=NASA |access-date=26 February 2022}}</ref>

== {{anchor|MWe|MWt|MWm|Conventions in the electric power industry}}Conventions in the electric power industry == <!-- linked from [[Advanced gas-cooled reactor]] -->
In the [[electric power industry]], ''megawatt electrical'' (''MWe''<ref>{{cite web |publisher= [[University of North Carolina at Chapel Hill]] |url= http://www.unc.edu/~rowlett/units/dictM.html |title= How Many? A Dictionary of Units of Measurement. M |first= Russ |last= Rowlett |access-date= 2017-03-04 |url-status= live |archive-url= https://web.archive.org/web/20110904024909/http://www.unc.edu/~rowlett/units/dictM.html |archive-date= 2011-09-04 }}</ref> or MW<sub>e</sub>)<ref>
{{cite encyclopedia | last =Cleveland | first = CJ |year=2007 |url= http://editors.eol.org/eoearth/wiki/Watt_(Energy) |title=Watt |encyclopedia= [[Encyclopedia of Earth]]}}</ref> refers by convention to the [[electric power]] produced by a generator, while ''megawatt thermal'' or ''thermal megawatt''<ref>
{{cite web
 |url         = http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=12362
 |title       = Solar Energy Grew at a Record Pace in 2008 (excerpt from EERE Network News
 |place       = [[United States of America|US]]
 |publisher   = Department of Energy)
 |date        = 25 March 2009
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20111018094231/http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=12362
 |archive-date = 18 October 2011
}}
</ref> (MWt, MW<sub>t</sub>, or MWth, MW<sub>th</sub>) refers to [[thermal power]] produced by the plant. For example, the [[Embalse nuclear power plant]] in Argentina uses a [[fission reactor]] to generate 2,109&nbsp;MW<sub>t</sub> (i.e. heat), which creates steam to drive a turbine, which generates 648&nbsp;MW<sub>e</sub> (i.e. electricity). Other [[SI prefix]]es are sometimes used, for example ''gigawatt electrical'' (GW<sub>e</sub>). The [[International Bureau of Weights and Measures]], which maintains the SI-standard, states that further information about a quantity should not be attached to the unit symbol but instead to the quantity symbol (e.g., {{nowrap|''P''<sub>th</sub> {{=}} 270 W}} rather than {{nowrap|''P'' {{=}} 270 W<sub>th</sub>}}) and so these unit symbols are non-SI.<ref>{{SIBrochure8th|page= 132}}</ref> In compliance with SI, the energy company [[Ørsted (company)|Ørsted A/S]] uses the unit megawatt for produced electrical power and the equivalent unit [[megajoule]] per second for delivered heating power in a [[combined heat and power]] station such as [[Avedøre Power Station]].<ref>{{cite web|url= http://www.dongenergy.com/en/business%20activities/generation/activities/central_power_stations/pages/avedoere_power_station.aspx |title= Avedøre Power Station (''Avedøre værket'') |publisher= [[DONG Energy]] |access-date= 2014-03-17 |url-status=dead |archive-url= https://web.archive.org/web/20140317181930/http://www.dongenergy.com/en/business%20activities/generation/activities/central_power_stations/pages/avedoere_power_station.aspx |archive-date= 2014-03-17 }}</ref>

When describing [[alternating current]] (AC) electricity, another distinction is made between the watt and the [[volt-ampere]]. While these units are equivalent for simple [[resistor|resistive]] [[Electrical network|circuit]]s, they differ when loads exhibit [[electrical reactance]].

==Radio transmission==
{{main|Effective radiated power}}
[[Radio stations]] usually report the power of their [[Radio transmitters|transmitters]] in units of watts, referring to the [[effective radiated power]]. This refers to the power that a [[Half-wave antenna|half-wave]] [[dipole antenna]] would need to radiate to match the intensity of the transmitter's [[main lobe]].

==Distinction between watts and watt-hours==
The terms [[Power (physics)|power]] and [[energy]] are closely related but distinct physical quantities. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy ''per unit time''.

For example, when a [[Electric light|light bulb]] with a [[power rating]] of {{gaps|100|W}} is turned on for one hour, the energy used is 100 [[watt hour]]s (W&middot;h), 0.1 kilowatt hour, or 360&nbsp;[[joule#Kilojoule|kJ]]. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.

[[Power station]]s are rated using units of power, typically megawatts or gigawatts (for example, the [[Three Gorges Dam]] in China, is rated at approximately 22 gigawatts). This reflects the maximum power output it can achieve at any point in time. A power station's annual energy output, however, would be recorded using units of energy (not power), typically gigawatt hours. Major energy production or consumption is often expressed as [[terawatt hour]]s for a given period; often a calendar year or financial year. One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or approximately 114 megawatts for a period of one year:

: Power output = energy / time

: 1 terawatt hour per year = 1&times;10<sup>12</sup> W·h / (365 days &times; 24 hours per day) ≈ 114 million watts,
equivalent to approximately 114 megawatts of constant power output.

The [[watt-second]] is a unit of energy, equal to the [[joule]]. One kilowatt hour is 3,600,000 watt seconds.

While a watt per hour is a unit of rate of change of power with time,{{efn-lr| Watts per hour refers to the ''rate of change'' of power being used (or generated). For example, a power plant that changes its power output from 100&nbsp;MW to 200&nbsp;MW in 15 minutes would have a ramp-up rate of 400&nbsp;MW/h. Gigawatts per hour are used to characterize the ramp-up required of the [[power plant]]s on an electric grid to compensate for loss of output from other sources, such as when [[solar power]] generation drops to zero as the sun sets. See [[duck curve]].}} it is not correct to refer to a watt (or watt-hour) as a watt per hour.<ref>{{cite web |url=http://www.windsun.com/Inverters/Inverter_selection.htm |title=Inverter Selection |publisher=Northern Arizona Wind and Sun |access-date=27 March 2009 |url-status=live |archive-url=https://web.archive.org/web/20090501140617/http://www.windsun.com/Inverters/Inverter_selection.htm |archive-date=1 May 2009 }}</ref>

==See also==
{{Portal|Energy|Engineering|Physics}}
* [[Kibble balance]] (formerly known as a watt balance)
* [[Nominal power (photovoltaic)]]
* [[Power factor]]
* [[Solar constant]]
* [[Conversion of units#Power or heat flow rate|Wattage conversion factors]]
* [[Wattmeter]]

== Explanatory notes ==
{{Notelist-lr}}

==References==
{{reflist}}

==External links==
{{Spoken Wikipedia|date=2023-07-18|Watt Wikipedia article spoken version.ogg}}
* {{Commons-inline}}
* {{Wiktionary-inline}}
* {{Cite web |last=Borvon |first=Gérard |url=http://seaus.free.fr/spip.php?article964 |title=History of the electrical units}}
* {{Cite book |last=Nelson |first=Robert A. |url=http://www.aticourses.com/international_system_units.htm |title=The International System of Units: Its History and Use in Science and Industry |series=Via Satellite |date=February 2000 |publisher=ATI courses}}

{{SI units}}

[[Category:James Watt]]
[[Category:SI derived units]]
[[Category:Units of power]]