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The watt (symbol: W) is a unit of
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
or
radiant flux In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
. In the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI), it is defined as a derived unit of (in
SI base units The SI base units are the standard units of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mat ...

SI base units
) 1 kg⋅m2⋅s−3 or, equivalently, 1
joule The joule ( ; symbol: J) is a derived unit of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates ...

joule
per
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
. It is used to quantify the rate of
energy transfer In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and fo ...
. The watt is named after
James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European lang ...

James Watt
(1736–1819), an 18th-century Scottish inventor.


Overview

When an object's
velocity The velocity of an object is the rate of change of its position with respect to a frame of reference In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical scie ...

velocity
is held constant at one
metre per second The metre per second is an SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...
against a constant opposing force of one
newton Newton most commonly refers to: * Isaac Newton (1642–1726/1727), English scientist * Newton (unit), SI unit of force named after Isaac Newton Newton may also refer to: Arts and entertainment * Newton (film), ''Newton'' (film), a 2017 Indian fil ...
, the rate at which
work Work may refer to: * Work (human activity) Work or labor is intentional activity people perform to support themselves, others, or the needs and wants of a wider community. Alternatively, work can be viewed as the human activity that cont ...

work
is done is one watt. : \mathrm In terms of
electromagnetism Electromagnetism is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in ...

electromagnetism
, one watt is the rate at which
electrical work Electric field work is the work performed by an electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), ...

electrical work
is performed when a current of one
ampere The ampere (, ; symbol: A), often shortened to "amp",SI supports only the use of symbols and deprecates the use of abbreviations for units. is the base unit of electric current An electric current is a stream of charged particle In physics ...

ampere
(A) flows across an electrical
potential difference Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is def ...

potential difference
of one
volt The volt is the derived unit for electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work (physics), work energy needed to move a ...

volt
(V), meaning the watt is equivalent to the
volt-ampere A volt-ampere ( SI symbol: V⋅A or V A; also VA) is the unit used for the apparent power Power (physics), Instantaneous power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating curre ...
(the latter unit, however, is used for a different quantity from the real power of an electrical circuit). : \mathrm Two additional unit conversions for watt can be found using the above equation and
Ohm's law Ohm's law states that the current Currents or The Current may refer to: Science and technology * Current (fluid) A current in a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an ap ...

Ohm's law
. : \mathrm where
ohm The ohm (symbol: Ω) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, ...

ohm
(\Omega) is the
SI derived unit SI derived units are units of measurement derived from the seven SI base unit, base units specified by the International System of Units (SI). They are either dimensionless quantity, dimensionless or can be expressed as a product of one or more o ...
of
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its Multiplicative inverse, reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares s ...
.


Examples

*A person having a mass of 100 kg who climbs a 3-metre-high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

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''. * 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.


Origin and adoption as an SI unit

The watt is named after the Scottish inventor
James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European lang ...

James Watt
. This unit name was proposed initially by 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 The British Science Association (BSA) is a charity Charity may refer to: Giving * Charitable organization or charity, a non-profit organization whose primary objectives are philanthropy and social well-being * Charity (practice), the practice ...
. Noting that units in the practical system of units were named after leading physicists, Siemens proposed that ''watt'' might be an appropriate name for a unit of power. Siemens defined the unit consistently within the then-existing system of practical units as "the power conveyed by a current of an
Ampère
Ampère
through the difference of potential of a Volt". In October 1908, at the International Conference on Electric Units and Standards in London, so-called "international" definitions were established for practical electrical units. Siemens' definition was adopted as the "international" watt. (Also used: 1 A2 × 1 Ω.) The watt was defined as equal to 107 units of power in the "practical system" of units. The "international units" were dominant from 1909 until 1948. After the 9th
General Conference on Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures The International Bureau of 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 now defined as the quantity of energy transferred in a unit of time, namely 1 J/s. In this new definition, 1 "absolute" watt = 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. In 1960, the 11th General Conference on Weights and Measures adopted the "absolute" watt into the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI) as the unit of power.


Multiples


Yoctowatt

The yoctowatt (yW) is equal to one septillionth (10−24) of a watt.


Zeptowatt

The zeptowatt (zW) is equal to one sextillionth (10−21) of a watt.


Attowatt

The attowatt (aW) is equal to one quintillionth (10−18) of a watt. The sound intensity in water corresponding to the international standard reference
sound pressure Sound pressure or acoustic pressure is the local pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can ca ...
of 1 μPa is approximately 0.65 aW/m2.


Femtowatt

The femtowatt (fW) is equal to one quadrillionth (10−15) of a watt. Technologically important powers that are measured in femtowatts are typically found in references to
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

radio
and
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
receivers. For example, meaningful
FM tuner A tuner is a subsystem that receives radio frequency Radio frequency (RF) is the oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and event (phil ...
performance figures for sensitivity, quieting and
signal-to-noise Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal (electrical engineering), signal to the level of background Noise (signal processing), noise. SNR is defined as the ratio o ...
require that the RF energy applied to the antenna input be specified. These input levels are often stated in dBf (
decibel The decibel (symbol: dB) is a relative unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude ( ...

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 The input impedance of an electrical network An electrical network is an interconnection of electronic component, electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a mode ...
of the tuner.


Picowatt

The picowatt (pW), not to be confused with the much larger petawatt (PW), is equal to one trillionth (10−12) of a watt. Technologically important powers that are measured in picowatts are typically used in reference to
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

radio
and radar receivers,
acoustics Acoustics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other wo ...
and in the science of
radio astronomy Radio astronomy is a subfield of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial ...
. One picowatt is the international standard reference value of
sound power Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. It is defined as "through a surface, the product of the sound pressure, and the component of the particle velocity, at ...
when this quantity is expressed as a level in
decibel The decibel (symbol: dB) is a relative unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude ( ...

decibel
s.


Nanowatt

The nanowatt (nW) is equal to one billionth (10−9) of a watt. Important powers that are measured in nanowatts are also typically used in reference to radio and radar receivers.


Microwatt

The microwatt (µW) is equal to one millionth (10−6) of a watt. Important powers that are measured in microwatts are typically stated in
medical instrument Artificial pacemaker, a Class III device in the United States A medical device is any device intended to be used for medical purposes. Medical devices benefit patients by helping health care providers diagnose and treat patients and helping pati ...
ation systems such as the
electroencephalograph Electroencephalography (EEG) is an electrophysiology, electrophysiological monitoring method to record electrical activity on the scalp that has been shown to represent the macroscopic activity of the surface layer of the brain underneath. It is ...
(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 A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived b ...

solar cells
for devices such as
calculators An electronic calculator is typically a portable Electronics, electronic device used to perform calculations, ranging from basic arithmetic to complex mathematics. The first Solid-state electronics, solid-state electronic calculator was created ...
and
watches A watch is a portable Clock, timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attach ...
are typically measured in microwatts.


Milliwatt

The milliwatt (mW) is equal to one thousandth (10−3) of a watt. A typical
laser pointer A laser pointer or laser pen is a small handheld device with a power source (usually a battery) and a laser diode emitting a very narrow Coherence (physics), coherent low-powered laser beam of visible light, intended to be used to highlight some ...

laser pointer
outputs about 5 milliwatts of light power, whereas a typical
hearing aid A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss Hearing loss is a partial or total inability to hear Hearing, or auditory perception, is the ability to perceive Sound, soun ...
uses less than 1 milliwatt.
Audio signal An audio signal is a representation of sound In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order ...
s and other electronic signal levels are often measured in
dBm dBm or dBmW (decibel-milliwatts) is a unit of level Level or levels may refer to: Engineering * Level (instrument), a device used to measure true horizontal or relative heights *Canal pound or level *Regrading or levelling, the process of rais ...
, referenced to 1 milliwatt.


Kilowatt

The kilowatt (kW) is equal to one thousand (103) watts. This unit is typically used to express the output power of
engine An engine or motor is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply For ...

engine
s and the power of , tools, machines, and heaters. It is also a common unit used to express the
electromagnetic Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

electromagnetic
power output of broadcast radio and television
transmitter In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particle ...
s. One kilowatt is approximately equal to 1.34
horsepower Horsepower (hp) is a unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the ...

horsepower
. A small electric heater with one
heating element A heating element converts electrical energy into heat through the process of Joule heating Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current An electric current ...
can use 1 kilowatt. The average electric power consumption of a household in the United States is about 1 kilowatt. A surface area of 1 square meter on Earth receives typically about 1 kilowatt of
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
from the Sun (the
solar irradiance Solar irradiance is the power (physics), power per unit area received from the Sun in the form of electromagnetic radiation as measured in the wavelength range of the measuring instrument. The solar irradiance is measured in watt per square met ...
) (on a clear day at mid day, close to the equator).


Megawatt

The megawatt (MW) is equal to one million (106) watts. 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 A server farm or server cluster is a collection of computer servers – usually maintained by an organization An organization, or organisation (Commonwealth English The use of the English language English is a of the , o ...
s or
data centers A data center (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Eng ...

data centers
; and some scientific research equipment, such as
supercollider , a synchrotron collider type particle accelerator at Fermi National Accelerator Laboratory (Fermilab), Batavia, Illinois, USA. Shut down in 2011, until 2007 it was the most powerful particle accelerator in the world, accelerating protons to a ...
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 locomotives An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a Battery (electricity), battery or a supercapacitor. Electric locomotives with on-board fueled prime mover (loco ...

electric locomotives
typically have a peak power output of or, while some produce much more. The
Eurostar Eurostar is an international high-speed rail service connecting the United Kingdom with France, Belgium and the Netherlands. Most Eurostar trains travel through the Channel Tunnel between the United Kingdom and France, owned and operated sep ...

Eurostar
, for example, uses more than , while heavy diesel-electric locomotives typically produce/use and. U.S.
nuclear power plant A nuclear power plant (sometimes abbreviated as NPP) is a thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel t ...

nuclear power plant
s have net summer capacities between about and. The earliest citing of the megawatt in the ''
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary A historical dictionary or dictionary on historical principles is a dictionary which deals not only with the latterday meanings of words but also the historica ...
'' (OED) is a reference in the 1900 . The OED also states that megawatt appeared in a 28 November 1947 article in the journal ''
Science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is something that is truth, true. The usual test for a statement of ...
'' (506:2).


Gigawatt

The gigawatt (GW) is equal to one billion (109) watts or 1 gigawatt = 1000 megawatts. This unit is often used for large power plants or power grids. For example, by the end of 2010, power shortages in China's
Shanxi Shanxi (; ; Chinese postal romanization, formerly romanised as Shansi) is a landlocked Provinces of China, province of the China, People's Republic of China and is part of the North China region. The capital and largest city of the province is ...

Shanxi
province were expected to increase to 5–6 GW and the installed capacity of wind power in Germany was 25.8 GW. The largest unit (out of four) of the Belgian
Doel Nuclear Power Station The Doel Nuclear Power Station is one of two nuclear power plants in Belgium Belgium ( nl, België ; french: Belgique ; german: Belgien ), officially the Kingdom of Belgium, is a country in Western Europe. It is bordered by the Netherlands ...
has a peak output of 1.04 GW.
HVDC converter A high-voltage, direct current (HVDC) electric power transmission Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used ...
s have been built with power ratings of up to 2 GW.


Terawatt

The terawatt (TW) is equal to one trillion (1012) watts. The total power used by humans worldwide is commonly measured in terawatts. The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for
nanosecond A nanosecond (ns) is an SI unit of time equal to one billionth of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités) ...
intervals. The average lightning strike peaks at 1 terawatt, but these strikes only last for 30
microsecond A microsecond is an SI unit of time equal to one millionth (0.000001 or 10−6 or ) of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système In ...
s.


Petawatt

The petawatt (PW) is equal to one quadrillion () watts and can be produced by the current generation of lasers for time scales on the order of picoseconds ( s). One such laser is the Lawrence Livermore's
Nova laser Nova was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) in California, United States, in 1984 which conducted advanced inertial confinement fusion (ICF) experiments until its dismantling in 1999. Nova was the first I ...
, which achieved a power output of 1.25 PW ( W) by a process called
chirped pulse amplification Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort pulse, ultrashort laser pulse up to the petawatt level, with the laser pulse being stretched out temporally and spectrally, then amplified, and then compressed again. The ...
. The duration of the pulse was roughly 0.5  ps ( s), giving a total energy of 600 J. Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE),
Osaka University , abbreviated as , is a public In public relations and communication science, publics are groups of individual people, and the public (a.k.a. the general public) is the totality of such groupings. This is a different concept to the sociolo ...
, which achieved a power output of 2 PW for a duration of approximately 1  ps. Based on the average
total solar irradiance Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...
of 1.366 kW/m2, the total power of sunlight striking Earth's atmosphere is estimated at 174 PW.


Conventions in the electric power industry

In the
electric power industry The electric power industry covers the generation A generation is "all of the people born and living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organism ...
, ''megawatt electrical'' (''MWe'' or MWe) refers by convention to the
electric power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loosely, ''electrical energy'' refers to energy that has been conve ...
produced by a generator, while ''megawatt thermal'' or ''thermal megawatt'' (MWt, MWt, or MWth, MWth) refers to
thermal power A thermal power station is a power station in which heat energy is converted to electricity. Typically, water is heated into steam, which is used to drive an electrical generator. After it passes through the turbine the steam is condensed in a s ...
produced by the plant. For example, the
Embalse nuclear power plant The Embalse Nuclear Power Station ( es, Central Nuclear Embalse) is one of three operational nuclear power plant A nuclear power plant (sometimes abbreviated as NPP) is a thermal power station in which the heat source is a nuclear reactor. As is ...
in Argentina uses a fission reactor to generate 2109 MWt (i.e. heat), which creates steam to drive a turbine, which generates 648 MWe (i.e. electricity). Other
SI prefix The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
es are sometimes used, for example ''gigawatt electrical'' (GWe). The
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation An intergovernmental organization (IGO) is an organization composed primarily of sovereign state ...
, 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 (i.e., ''P''thermal = 270 W rather than ''P'' = 270 Wth) and so these units are non-SI. In compliance with SI, the energy company Ørsted A/S uses the unit megawatt for produced electrical power and the equivalent unit
megajoule The joule ( ; symbol: J) is a derived unit of energy In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to a physical body, body or physical system to perform W ...
per second for delivered heating power in a
combined heat and power Cogeneration or combined heat and power (CHP) is the use of a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, ...
station such as Avedøre Power Station. ''Megawatt mechanical'' (''MWm)'' is rarely used. When describing
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
(AC) electricity, another distinction is made between the watt and the
volt-ampere A volt-ampere ( SI symbol: V⋅A or V A; also VA) is the unit used for the apparent power Power (physics), Instantaneous power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating curre ...
. While these units are equivalent for simple
resistive In electronics and electromagnetism, the electrical resistance of an object is a measure of its opposition to the flow of electric current. The Multiplicative inverse, reciprocal quantity is , and is the ease with which an electric current passe ...

resistive
circuitCircuit may refer to: Science and technology Electrical engineering * Electrical circuit, a complete electrical network with a closed-loop giving a return path for current ** Analog circuit, uses continuous signal levels ** Balanced circuit, p ...
s, they differ when loads exhibit
electrical reactance In electrical circuits, reactance is the opposition presented to alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct cu ...

electrical reactance
.


Radio transmission

Radio stations Radio broadcasting is transmission of audio Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is th ...
usually report the power of their
transmitters In electronics and telecommunications a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which is applied ...
in units of watts, referring to the
effective radiated power Effective radiated power (ERP), synonymous with equivalent radiated power, is an IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a professional association for electronic engineering and electrical engineering (and ass ...
. This refers to the power that a half-wave
dipole antenna In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna (radio), antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary el ...

dipole antenna
would need to radiate to match the intensity of the transmitter's
main lobe In a antenna (electronics), radio antenna's radiation pattern, the main lobe, or main beam, is the Side lobe, lobe containing the higher Power (physics), power. This is the lobe that exhibits the greater field strength. The radiation pattern of ...

main lobe
.


Distinction between watts and watt-hours

The terms
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
and
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

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
light bulb An electric light is a device that produces visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum ...
with a
power rating In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch of p ...
of is turned on for one hour, the energy used is 100 
watt hour The kilowatt-hour (International System of Units, SI symbol: kW⋅h or kW h; commonly written as kWh) is a Units of measurement, unit of energy equal to one kilowatt of Power (physics), power sustained for one hour or 3600 kilojoules ...
s (W·h), 0.1 kilowatt hour, or 360  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 stations A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...

Power stations
are rated using units of power, typically megawatts or gigawatts (for example, the
Three Gorges Dam The Three Gorges Dam is a hydroelectric Hydroelectricity, or hydroelectric power, is electricity produced from hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running ...

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 The kilowatt-hour ( SI symbol: kW⋅h or kW h; commonly written as kWh) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, ...
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 = 1×1012 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 The joule ( ; symbol: J) is a derived unit of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates ...

joule
. One kilowatt hour is 3,600,000 watt seconds. While a watt per hour exists in principle (as a unit of rate of change of power with time), it is not correct to refer to a watt (or watt hour) as a "watt per hour".


See also

*
Kibble balance Kibble may refer to: * Dry compound feed, especially when used as dog food or cat food * chalk and flint rubble, also known as kibble in East Devon, used to consolidate ground * a large Bucket (machine part), bucket, as used to raise ore from a mine ...
(formerly known as a watt balance) *
Nominal power (photovoltaic) The nominal power is the nameplate capacity of photovoltaic (PV) devices, such as solar cell A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light Light or visible light is electromagnetic radi ...
*
Power factor In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch of ph ...
*
Solar constant The solar constant (''GSC'') is a flux density measuring mean solar electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature') ...
*
Volt-ampere A volt-ampere ( SI symbol: V⋅A or V A; also VA) is the unit used for the apparent power Power (physics), Instantaneous power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating curre ...
* Wattage conversion factors *
Wattmeter The wattmeter is an instrument for measuring the electric power Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt The watt (symbol: W) is a unit ...

Wattmeter
*
Primary energy Primary energy (PE) is an energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nat ...


Explanatory notes


References


External links

* * {{SI units SI derived units Units of power James Watt