View source for 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]]}}
{{Use American English|date=September 2024}}
{{Use mdy dates|date=September 2024}}

{{Infobox Unit
| bgcolour =
| name = watt
| image = Alimentation 420 W.jpg
| caption = A 420-watt [[power supply]]
| standard = [[SI]]
| quantity = [[Power (physics)|power]]
| symbol = W
| namedafter = [[James Watt]]
| units1 = [[SI base unit]]s
| inunits1 = 1 [[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}}
}}
{{Electromagnetism|Network}}

The '''watt''' (symbol: '''W''') is the unit of [[Power (physics)|power]] or [[radiant flux]] in the [[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]] [[inventor]], [[mechanical engineer]], and [[chemist]] who improved the [[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 {\cdot} A}.</math>

Two additional [[unit conversion]]s 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 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 book|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=April 9, 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]] require that the [[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 [[electroencephalograph]] (EEG) and the [[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 = July 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 = March 21, 2017 }}.</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 = February 8, 2010 | url-status = live | archive-url = https://web.archive.org/web/20110303094710/http://www.datasheetarchive.com/datasheet-pdf/019/DSA00333218.html | archive-date = March 3, 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 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    = February 14, 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   = January 9, 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 [[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 = [[United States Nuclear Regulatory Commission]] | language = en-us | date = August 1, 2007 | 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 = December 27, 2021 | 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 = February 16, 2008 | 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 November 28, 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 [[power grid]]s. 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 | archive-url = https://web.archive.org/web/20201121173708/https://in.reuters.com/article/idINTOE6AA0AD20101111 | url-status = dead | archive-date = November 21, 2020 | title = China's Shanxi to face 5–6 GW power shortage by yr-end – paper | date = November 11, 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=August 19, 2010 | url-status=live | archive-url=https://web.archive.org/web/20100824080835/http://www.economist.com/node/16846774 | archive-date=August 24, 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=July 10, 2011}}</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=February 9, 2020}}</ref> [[Earth's internal heat budget|Earth]] itself emits 47±2 TW,<ref>{{Cite journal |last1=Davies |first1=J. H. |last2=Davies |first2=D. R. |date=February 22, 2010 |title=Earth's surface heat flux |url=https://se.copernicus.org/articles/1/5/2010/ |journal=Solid Earth |language=English |volume=1 |issue=1 |pages=5–24 |doi=10.5194/se-1-5-2010 |doi-access=free |bibcode=2010SolE....1....5D |issn=1869-9510}}</ref> far less than the energy received from solar radiation.  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>:{{See also|List of petawatt lasers}} 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]], 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 = June 19, 2012 |url-status = live |archive-url = https://web.archive.org/web/20120915212555/https://www.llnl.gov/str/Petawatt.html |archive-date = September 15, 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 = August 12, 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 = August 22, 2015 }}.</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 = August 8, 2015 }}.</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 = October 5, 2005 | url-status = live | archive-url = https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant | archive-date = August 30, 2011 }}</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=June 15, 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=February 26, 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= March 4, 2017 |url-status= live |archive-url= https://web.archive.org/web/20110904024909/http://www.unc.edu/~rowlett/units/dictM.html |archive-date= September 4, 2011 }}</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       = [[US]]
 |publisher   = Department of Energy)
 |date        = March 25, 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 = October 18, 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= March 17, 2014 |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= March 17, 2014 }}</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 [[transmitter]]s 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 {{em|per unit time}}.

For example, when a [[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·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 = {{val|1e12|u=W·h}} / (365 days × 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 {{em|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=March 27, 2009 |url-status=live |archive-url=https://web.archive.org/web/20090501140617/http://www.windsun.com/Inverters/Inverter_selection.htm |archive-date=May 1, 2009 }}</ref>

==See also==
{{Portal|Energy|Engineering|Physics}}
* [[Kibble balance]] (formerly known as a watt balance)
* [[Nominal power (photovoltaic)]]
* [[One Watt Initiative]]
* [[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=July 18, 2023|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]]