The watt (symbol: W) is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second,[1] and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by .[2]

Unit systemSI derived unit
Unit ofPower
Named afterJames Watt
1 W in ...... is equal to ...
   SI base units   kgm2s−3
   CGS units   1×107 erg s−1


When an object's velocity is held constant at one meter per second against a constant opposing force of one newton, the rate at which work is done is 1 watt.

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).

Two additional unit conversions for watt can be found using the above equation and Ohm's Law.

Where ohm () is the SI derived unit of electrical resistance.

  • A person having a mass of 100 kilograms 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.[i]
  • A laborer over the course of an 8-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.[3]

Origin and adoption as an SI unit

The watt is named after the Scottish inventor James Watt.[4] This unit 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.[5] 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.[6] Siemens defined the unit consistently within the then-existing system of practical units as "the power conveyed by a current of an Ampère through the difference of potential of a Volt."[7]

In October 1908, at the International Conference on Electric Units and Standards in London,[8] so-called "international" definitions were established for practical electrical units.[9] Siemens' definition was adopted as the "international" watt. (Also used: 1 ampere2 x 1 ohm.)[4] The watt was defined as equal to 107 units of power in the "practical system" of units.[9] The "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 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.[4] 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.[10]


For additional examples of magnitude for multiples and submultiples of the watt, see Orders of magnitude (power)
SI multiples of watt (W)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1 W dW deciwatt 101 W daW decawatt
10−2 W cW centiwatt 102 W hW hectowatt
10−3 W mW milliwatt 103 W kW kilowatt
10−6 W µW microwatt 106 W MW megawatt
10−9 W nW nanowatt 109 W GW gigawatt
10−12 W pW picowatt 1012 W TW terawatt
10−15 W fW femtowatt 1015 W PW petawatt
10−18 W aW attowatt 1018 W EW exawatt
10−21 W zW zeptowatt 1021 W ZW zettawatt
10−24 W yW yoctowatt 1024 W YW yottawatt
Common multiples are in bold face


The attowatt (aW) is equal to 10−18 watt. The sound intensity in water corresponding to the international standard reference sound pressure of 1 μPa is approximately 0.65 aW/m2.[11]


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 reference(s) 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 (decibels referenced to 1 femtowatt). This is 0.2739 microvolt 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.


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 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 as a level in decibels.[12]


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.


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 instrumentation systems such as the EEG and the 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 calculators and watches are typically measured in microwatts.[13]


The milliwatt (mW) is equal to one thousandth (10−3) of a watt. A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid for people uses less than one milliwatt.[14] Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.


The kilowatt (kW) is equal to one thousand (103) watts. This unit is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters.

One kilowatt is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average electric power consumption of a household in the United States is about one kilowatt.[ii]

A surface area of one square meter on Earth receives typically about one kilowatt of sunlight from the sun (the solar irradiance) (on a clear day at mid day, close to the equator).[16]


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 farms or data centers; and some scientific research equipment, such as supercolliders, 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 typically have a peak power output of 5 or 6 MW, although some produce much more. The Eurostar, for example, uses more than 12 MW, while heavy diesel-electric locomotives typically produce/use 3 to 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.[17]

The earliest citing of the megawatt in the Oxford English Dictionary (OED) is a reference in the 1900 Webster's International Dictionary of English Language. The OED also states that megawatt appeared in a 28 November 1947 article in the journal Science (506:2).


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 province were expected to increase to 5–6 GW[18] and the installed capacity of wind power in Germany was 25.8 GW.[19] The largest unit (out of four) of the Belgian Doel Nuclear Power Station has a peak output of 1.04 GW.[20] HVDC converters have been built with power ratings of up to 2 GW.[21]


The terawatt (TW) is equal to one trillion (1012) watts. The total power used by humans worldwide is commonly measured in terawatts (see primary energy). The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for nanosecond time frames. The average lightning strike peaks at 1 terawatt, but these strikes only last for 30 microseconds.


The petawatt (PW) is equal to one quadrillion (1015) watts and can be produced by the current generation of lasers for time scales on the order of picoseconds (1012 s). One such laser is the Lawrence Livermore's Nova laser, which achieved a power output of 1.25 PW (1.25×1015 W) by a process called chirped pulse amplification. The duration of the pulse was roughly 0.5 ps (5×10−13 s), giving a total energy of 600 J.[22] 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 PW for a duration of approximately 1 ps.[23][24]

Based on the average total solar irradiance[25] of 1.366 kW/m2, the total power of sunlight striking Earth's atmosphere is estimated at 174 PW (see: solar constant).

Conventions in the electric power industry

In the electric power industry, megawatt electrical (MWe[26] or MWe[27]) refers by convention to the electric power produced by a generator, while megawatt thermal or thermal megawatt[28] (MWt, MWt, or MWth, MWth) refers to thermal power produced by the plant. For example, the Embalse nuclear power plant 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 prefixes are sometimes used, for example gigawatt electrical (GWe). 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 (i.e., Pthermal = 270 W rather than P = 270 Wth) and so these units are non-SI.[29] In compliance with SI the energy company DONG Energy uses the unit megawatt for produced electrical power and the equivalent unit megajoule/s for delivered heating power in a combined heat and power station such as Avedøre Power Station.[30]

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

Radio transmission

Radio stations usually report the power of their transmitters in units of watts, referring to the effective radiated power. This refers to the power that a 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 and energy are frequently confused. 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 with a power rating of 100W is turned on for one hour, the energy used is 100 watt hours (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 are rated using units of power, typically megawatts or gigawatts (for example, the Three Gorges Dam 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 hours 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 Wh / (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 exists in principle (as a unit of rate of change of power with time[iii]), it is not correct to refer to a watt (or watt hour) as a "watt per hour".[31]

See also


  1. ^ The energy in climbing the stairs is given by mgh. Setting m = 100 kg, g = 9.8 m/s2 and h = 3 m gives 2940 J. Dividing this by the time taken (5 s) gives a power of 588 W.
  2. ^ Average household electric power consumption is 1.19 kW in the US, 0.53 kW in the UK. In India it is 0.13 kW (urban) and 0.03 kW (rural) – computed from GJ figures quoted by Nakagami, Murakoshi and Iwafune.[15]
  3. ^ Watts per hour would properly refer to a rate of change of power being used (or generated). Watts per hour might be useful to characterize the ramp-up behavior of power plants, or slow-reacting plant where their power could only change slowly. For example, a power plant that changes its power output from 1 MW to 2 MW in 15 minutes would have a ramp-up rate of 4 MW/h.


  1. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), pp. 118, 144, ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
  2. ^ Yildiz, I.; Liu, Y. (2018). "Energy units, conversions, and dimensional analysis". In Dincer, I. (ed.). Comprehensive energy systems. Vol 1: Energy fundamentals. Elsevier. pp. 12–13. ISBN 9780128149256.
  3. ^ Avallone, Eugene A; et al., eds. (2007), Marks' Standard Handbook for Mechanical Engineers (11th ed.), New York: Mc-Graw Hill, pp. 9–4, ISBN 0-07-142867-4 Explicit use of et al. in: |editor2= (help).
  4. ^ a b c Klein, H.A. (1988) [1974]. The science of measurement: A historical survey. New York: Dover. p. 239. ISBN 9780486144979.
  5. ^ "Address by C. William Siemens". Report of the Fifty-Second meeting of the British Association for the Advancement of Science. London: John Murray. 1883. pp. 1–33.
  6. ^ 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." "Siemens, 1883, p. 6"
  7. ^ "Siemens", 1883, p. 5"
  8. ^ Tunbridge, P. (1992). Lord Kelvin: His Influence on Electrical Measurements and Units. Peter Peregrinus: London. p. 51. ISBN 0-86341-237-8.
  9. ^ a b "Units, Physical". Encyclopædia Britannica. 27 (11th ed.). 1911. p. 742.
  10. ^ "Resolution 12 of the 11th CGPM (1960)". Bureau International des Poids et Mesures (BIPM). Retrieved 9 April 2018.
  11. ^ Ainslie, M. A. (2015). A century of sonar: Planetary oceanography, underwater noise monitoring, and the terminology of underwater sound. Acoustics Today.
  12. ^ Morfey, C.L. (2001). Dictionary of Acoustics.
  13. ^ "Bye-Bye Batteries: Radio Waves as a Low-Power Source", The New York Times, Jul 18, 2010, archived from the original on 2017-03-21.
  14. ^ Stetzler, Trudy; Magotra, Neeraj; Gelabert, Pedro; Kasthuri, Preethi; Bangalore, Sridevi. "Low-Power Real-Time Programmable DSP Development Platform for Digital Hearing Aids". Datasheet Archive. Archived from the original on 3 March 2011. Retrieved 8 February 2010.
  15. ^ Nakagami, Hidetoshi; Murakoshi, Chiharu; Iwafune, Yumiko (2008). International Comparison of Household Energy Consumption and Its Indicator (PDF). ACEEE Summer Study on Energy Efficiency in Buildings. Pacific Grove, California: American Council for an Energy-Efficient Economy. Figure 3. Energy Consumption per Household by Fuel Type. 8:214–8:224. Archived (PDF) from the original on 9 January 2015. Retrieved 14 February 2013.
  16. ^ Elena Papadopoulou, Photovoltaic Industrial Systems: An Environmental Approach Springer 2011 ISBN 3642163017, p.153
  17. ^ "2007–2008 Information Digest, Appendix A" (PDF). Nuclear Regulatory Commission. 2007. Archived (PDF) from the original on 16 February 2008. Retrieved 27 January 2008.
  18. ^ Bai, Jim; Chen, Aizhu (11 November 2010). Lewis, Chris (ed.). "China's Shanxi to face 5–6 GW power shortage by yr-end – paper". Peking: Reuters.
  19. ^ "Not on my beach, please". The Economist. 19 August 2010. Archived from the original on 24 August 2010.
  20. ^ "Chiffres clés" [Key numbers]. Electrabel. Who are we: Nuclear (in French). 2011. Archived from the original on 2011-07-10.
  21. ^ Davidson, CC; Preedy, RM; Cao, J; Zhou, C; Fu, J (October 2010), "Ultra-High-Power Thyristor Valves for HVDC in Developing Countries", 9th International Conference on AC/DC Power Transmission, London: IET.
  22. ^ "Crossing the Petawatt threshold". Livermore, CA: Lawrence Livermore National Laboratory. Archived from the original on 15 September 2012. Retrieved 19 June 2012.
  23. ^ World’s most powerful laser: 2 000 trillion watts. What’s it?, IFL Science, archived from the original on 2015-08-22.
  24. ^ Eureka alert (publicity release), Aug 2015, archived from the original on 2015-08-08.
  25. ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". CH: PMODWRC. Archived from the original on 2011-08-22. Retrieved 2005-10-05.
  26. ^ Rowlett, Russ. "How Many? A Dictionary of Units of Measurement. M". University of North Carolina at Chapel Hill. Archived from the original on 2011-08-22. Retrieved 2017-03-04.
  27. ^ Cleveland, CJ (2007). "Watt". Encyclopedia of Earth.
  28. ^ "Solar Energy Grew at a Record Pace in 2008 (excerpt from EERE Network News". US: Department of Energy). 25 March 2009. Archived from the original on 18 October 2011.
  29. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 132, ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
  30. ^ "Avedøre Power Station (Avedøre værket)". DONG Energy. Archived from the original on 2014-03-17. Retrieved 2014-03-17.
  31. ^ "Inverter Selection". Northern Arizona Wind and Sun. Archived from the original on 1 May 2009. Retrieved 27 March 2009.

External links

Boulton and Watt

Boulton & Watt was an early British engineering and manufacturing firm in the business of designing and making marine and stationary steam engines. Founded in the English West Midlands around Birmingham in 1775 as a partnership between the English manufacturer Matthew Boulton and the Scottish engineer James Watt, the firm had a major role in the Industrial Revolution and grew to be a major producer of steam engines in the 19th century.

Cost of electricity by source

The distinct ways of electricity generation can incur significantly different costs. Calculations of these costs can be made at the point of connection to a load or to the electricity grid. The cost is typically given per kilowatt-hour or megawatt-hour. It includes the initial capital, discount rate, as well as the costs of continuous operation, fuel, and maintenance. This type of calculation assists policymakers, researchers and others to guide discussions and decision making.

The levelized cost of energy (LCOE) is a measure of a power source that allows comparison of different methods of electricity generation on a consistent basis. It is an economic assessment of the average total cost to build and operate a power-generating asset over its lifetime divided by the total energy output of the asset over that lifetime. The LCOE can also be regarded as the average minimum price at which electricity must be sold in order to break-even over the lifetime of the project.

Heriot-Watt University

Heriot-Watt University is a public university based in Edinburgh, Scotland. Originally established in 1821 as the School of Arts of Edinburgh, the world's first mechanics' institute, it was granted university status by royal charter granted in 1966. It has campuses in Edinburgh, the Scottish Borders, Orkney, United Arab Emirates and Putrajaya in Malaysia. It takes the name Heriot-Watt from Scottish inventor James Watt and Scottish philanthropist and goldsmith George Heriot.Heriot-Watt was named International University of the Year by The Times and Sunday Times Good University Guide 2018. The university is ranked among 302 in 2019 QS World University Rankings, 351-400 in 2019 Times Higher Education World University Rankings. In 2019, The Complete University Guide ranked Heriot-Watt at 35 in UK, ahead of several Russell Group universities. In the latest Research Excellence Framework, it was ranked overall in the Top 25% of UK universities and 1st in Scotland for research impact.


Horsepower (hp) is a unit of measurement of power, or the rate at which work is done. There are many different standards and types of horsepower. Two common definitions being used today are the mechanical horsepower (or imperial horsepower), which is about 745.7 watts, and the metric horsepower, which is approximately 735.5 watts.

The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery. The definition of the unit varied among geographical regions. Most countries now use the SI unit watt for measurement of power. With the implementation of the EU Directive 80/181/EEC on January 1, 2010, the use of horsepower in the EU is permitted only as a supplementary unit.

J. J. Watt

Justin James Watt (born March 22, 1989) is an American football defensive end for the Houston Texans of the National Football League (NFL). He was drafted by the Texans with the 11th pick in the first round of the 2011 NFL Draft, and played college football at Wisconsin.

Watt received the AP NFL Defensive Player of the Year Award three times in his first five seasons. Although primarily a defensive end, he occasionally shifts to defensive tackle in some situations. He has also taken snaps on offense, catching three touchdown passes during the 2014 season. In 2014, Watt became the first player in NFL history to record two 20+ sack seasons in a career. He holds the Texans' franchise records for both sacks and forced fumbles. In 2017, Sports Illustrated named Watt its Sportsperson of the Year.

James Watt

James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.

While working as an instrument maker at the University of Glasgow, Watt became interested in the technology of steam engines. He realised that contemporary engine designs wasted a great deal of energy by repeatedly cooling and reheating the cylinder. Watt introduced a design enhancement, the separate condenser, which avoided this waste of energy and radically improved the power, efficiency, and cost-effectiveness of steam engines. Eventually he adapted his engine to produce rotary motion, greatly broadening its use beyond pumping water.

Watt attempted to commercialise his invention, but experienced great financial difficulties until he entered a partnership with Matthew Boulton in 1775. The new firm of Boulton and Watt was eventually highly successful and Watt became a wealthy man. In his retirement, Watt continued to develop new inventions though none was as significant as his steam engine work.

He developed the concept of horsepower, and the SI unit of power, the watt, was named after him.


The joule (/dʒuːl/; symbol: J) is a derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre (1 newton metre or N⋅m). It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).

In terms firstly of base SI units and then in terms of other SI units:

where kg is the kilogram, m is the metre, s is the second, N is the newton, Pa is the pascal, W is the watt, C is the coulomb, and V is the volt.

One joule can also be defined as:

Kilowatt hour

The kilowatt hour (symbol kWh, kW⋅h or kW h) is a unit of energy equal to 3.6 megajoules. If energy is transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt hours is equal to the power in kilowatts multiplied by the time in hours. The kilowatt hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.

Mike Watt

Michael David Watt (born December 20, 1957) is an American bassist, vocalist and songwriter.

He is best known for co-founding and playing bass guitar for the rock bands Minutemen, Dos, and Firehose. He is also the frontman for the supergroup Big Walnuts Yonder, a member of the art rock group Banyan and involved with several other musical projects. From 2003 until 2013, he was the bass guitarist for The Stooges.

CMJ New Music called Watt a "seminal post-punk bass player." Readers of NME voted Mike Watt one of the "40 Greatest Bassists Of All Time" and LA Weekly awarded him the number six spot in "The 20 Best Bassists of All Time."In November 2008, Watt received the Bass Player Magazine lifetime achievement award, presented by Flea.

Minutemen (band)

Minutemen were an American punk rock band formed in San Pedro, California in 1980. Composed of guitarist/vocalist D. Boon, bassist/vocalist Mike Watt, and drummer George Hurley, Minutemen recorded four albums and eight EPs before Boon's death in an automobile accident in 1985; after Boon's death, the band broke up. They were noted in the California punk community for a philosophy of "jamming econo"—a sense of thriftiness reflected in their touring and presentation—while their eclectic and experimental attitude was instrumental in pioneering alternative rock and post-hardcore.


Muhammad (Arabic: مُحمّد‎, pronounced [muħammad]; c. 570 CE – 8 June 632 CE) was the founder of Islam. According to Islamic doctrine, he was a prophet, sent to present and confirm the monotheistic teachings preached previously by Adam, Abraham, Moses, Jesus, and other prophets. He is viewed as the final prophet of God in all the main branches of Islam, though some modern denominations diverge from this belief. Muhammad united Arabia into a single Muslim polity, with the Quran as well as his teachings and practices forming the basis of Islamic religious belief.

Born approximately 570 CE (Year of the Elephant) in the Arabian city of Mecca, Muhammad was orphaned at the age of six. He was raised under the care of his paternal grandfather Abd al-Muttalib, and upon his death, by his uncle Abu Talib. In later years he would periodically seclude himself in a mountain cave named Hira for several nights of prayer. When he was 40, Muhammad reported being visited by Gabriel in the cave, and receiving his first revelation from God. Three years later, in 610, Muhammad started preaching these revelations publicly, proclaiming that "God is One", that complete "submission" (islām) to God is the right way of life (dīn), and that he was a prophet and messenger of God, similar to the other prophets in Islam.The followers of Muhammad were initially few in number, and experienced hostility from Meccan polytheists. He sent some of his followers to Abyssinia in 615 to shield them from prosecution, before he and his followers migrated from Mecca to Medina (then known as Yathrib) in 622. This event, the Hijra, marks the beginning of the Islamic calendar, also known as the Hijri Calendar. In Medina, Muhammad united the tribes under the Constitution of Medina. In December 629, after eight years of intermittent fighting with Meccan tribes, Muhammad gathered an army of 10,000 Muslim converts and marched on the city of Mecca. The conquest went largely uncontested and Muhammad seized the city with little bloodshed. In 632, a few months after returning from the Farewell Pilgrimage, he fell ill and died. By the time of his death, most of the Arabian Peninsula had converted to Islam.The revelations (each known as Ayah, lit. "Sign [of God]"), which Muhammad reported receiving until his death, form the verses of the Quran, regarded by Muslims as the verbatim "Word of God" and around which the religion is based. Besides the Quran, Muhammad's teachings and practices (sunnah), found in the Hadith and sira (biography) literature, are also upheld and used as sources of Islamic law (see Sharia).

Performance per watt

In computing, performance per watt is a measure of the energy efficiency of a particular computer architecture or computer hardware. Literally, it measures the rate of computation that can be delivered by a computer for every watt of power consumed. This rate is typically measured by performance on the LINPACK benchmark when trying to compare between computing systems.

System designers building parallel computers, such as Google's hardware, pick CPUs based on their (other than Green500) performance per watt of power, because the cost of powering the CPU outweighs the cost of the CPU itself.

Power (physics)

In physics, power is the rate of doing work or of transferring heat, i.e. the amount of energy transferred or converted per unit time. Having no direction, it is a scalar quantity. In the International System of Units, the unit of power is the joule per second (J/s), known as the watt in honour of James Watt, the eighteenth-century developer of the condenser steam engine. Another common and traditional measure is horsepower (comparing to the power of a horse). Being the rate of work, the equation for power can be written:

As a physical concept, power requires both a change in the physical system and a specified time in which the change occurs. This is distinct from the concept of work, which is only measured in terms of a net change in the state of the physical system. The same amount of work is done when carrying a load up a flight of stairs whether the person carrying it walks or runs, but more power is needed for running because the work is done in a shorter amount of time.

The output power of an electric motor is the product of the torque that the motor generates and the angular velocity of its output shaft. The power involved in moving a vehicle is the product of the traction force of the wheels and the velocity of the vehicle. The rate at which a light bulb converts electrical energy into light and heat is measured in watts—the higher the wattage, the more power, or equivalently the more electrical energy is used per unit time.

Robert Watson-Watt

Sir Robert Alexander Watson-Watt, KCB, FRS, FRAeS (13 April 1892 – 5 December 1973) was a Scottish pioneer of radio direction finding and radar technology.Watt began his career in radio physics with a job at the Met Office, where he began looking for ways to accurately track thunderstorms using the radio signals given off by lightning. This led to the 1920s development of a system later known as huff-duff. Although well publicized at the time, the system's enormous military potential was not developed until the late 1930s. Huff-duff allowed operators to determine the location of an enemy radio in seconds and it became a major part of the network of systems that helped defeat the U-boat threat. It is estimated that huff-duff was used in about a quarter of all attacks on U-boats.

In 1935 Watt was asked to comment on reports of a German death ray based on radio. Watt and his assistant Arnold Frederic Wilkins quickly determined it was not possible, but Wilkins suggested using radio signals to locate aircraft at long distances. This led to a February 1935 demonstration where signals from a BBC short-wave transmitter were bounced off a Handley Page Heyford aircraft. Watt led the development of a practical version of this device, which entered service in 1938 under the code name Chain Home. This system provided the vital advance information that helped the Royal Air Force win the Battle of Britain.After the success of his invention, Watson-Watt was sent to the US in 1941 to advise on air defence after Japan’s attack on Pearl Harbor. He returned and continued to lead radar development for the War Ministry and Ministry of Supply. He was elected a Fellow of the Royal Society in 1941, was given a knighthood in 1942 and was awarded the US Medal for Merit in 1946.

SI derived unit

SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation.

The SI has special names for 22 of these derived units (for example, hertz, the SI unit of measurement of frequency), but the rest merely reflect their derivation: for example, the square metre (m2), the SI derived unit of area; and the kilogram per cubic metre (kg/m3 or kg m−3), the SI derived unit of density.

The names of SI derived units, when written in full, are in lowercase. However, the symbols for units named after persons are written with an uppercase initial letter. For example, the symbol for hertz is "Hz"; but the symbol for metre is "m".

T. J. Watt

Trent Jordan "T. J." Watt (born October 11, 1994) is an American football outside linebacker for the Pittsburgh Steelers of the National Football League (NFL). He played college football at the University of Wisconsin, and was drafted by the Steelers in the first round of the 2017 NFL Draft. His older brothers are J. J. Watt of the Houston Texans and Derek Watt of the Los Angeles Chargers.

Thermal conductivity

The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by , , or .

Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials like Styrofoam. Correspondingly, materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal conductivity is called thermal resistivity.

The defining equation for thermal conductivity is , where is the heat flux, is the thermal conductivity, and is the temperature gradient. This is known as Fourier's Law for heat conduction. Although commonly expressed as a scalar, the most general form of thermal conductivity is a second-rank tensor. However, the tensorial description only becomes necessary in materials which are anisotropic.

W. Montgomery Watt


William Montgomery Watt (14 March 1909 – 24 October 2006) was a Scottish historian, Orientalist, Anglican priest, and academic. From 1964 to 1979, he was Professor of Arabic and Islamic studies at the University of Edinburgh.

Watt was one of the foremost non-Muslim interpreters of Islam in the West, and according to Carole Hillenbrand "an enormously influential scholar in the field of Islamic studies and a much-revered name for many Muslims all over the world". Watt's comprehensive biography of the Islamic prophet Muhammad, Muhammad at Mecca (1953) and Muhammad at Medina (1956), are considered to be classics in the field.

Watt steam engine

The Watt steam engine, alternatively known as the Boulton and Watt steam engine, was the first practical steam engine and was one of the driving forces of the industrial revolution. James Watt developed the design sporadically from 1763 to 1775 with support from Matthew Boulton. Watt's design saved significantly more fuel compared to earlier designs that they were licensed based on the amount of fuel they would save. Watt never ceased developing the steam engine, introducing double-acting designs (with two cylinders) and various systems for taking off rotary power. Watt's design became synonymous with steam engines, and it was many years before significantly new designs began to replace the basic Watt design.

The first steam engines, introduced by Thomas Newcomen in 1712, were of the "atmospheric" design. Steam was introduced into a cylinder which was then cooled by a spray of water. This caused the steam to condense, forming a partial vacuum in the cylinder. Atmospheric pressure on the top pushed the piston down. Watt noticed that the water spray also cooled the cylinder itself, and it required significant amounts of heat to warm it back up to the point where steam could enter the cylinder without immediately condensing again. Watt addressed this by adding a separate water-filled cylinder which was opened once the main cylinder was filled. The steam entered the secondary cylinder and condensed, drawing remaining steam from the main cylinder to continue the process. The end result was the same cycle as Newcomen's design, but without any cooling of the main cylinder which was immediately ready for another stroke. Watt worked on the design over a period of several years, introducing the condenser and improvements to practically every part of the design, notably a lengthy series of trials on ways to seal the piston in the cylinder. All of these changes produced a more reliable design which used half as much coal to produce the same amount of power.The new design was introduced commercially in 1776, with the first example sold to the Carron Company ironworks. Watt continued working to improve the engine, and in 1781 introduced a system using a sun and planet gear to turn the linear motion of the engines into rotary motion. This made it useful not only in the original pumping role, but also as a direct replacement in roles where a water wheel would have been used previously. This was a key moment in the industrial revolution, since power sources could now be located anywhere instead of, as previously, needing a suitable water source and topography. Boulton began developing a multitude of machines that made use of this rotary power, developing the first modern industrialized factory, the Soho Foundry, which in turn produced new steam engine designs. Watt's early engines were like the original Newcomen designs in that they used low-pressure steam and most of the action was caused by atmospheric pressure, due mostly to safety concerns. Looking to improve on their performance, Watt began considering the use of higher-pressure steam, as well as designs using multiple cylinders in both the double-acting concept and the multiple-expansion concept. These double-acting engines required the invention of the parallel motion, which allowed the piston rods of the individual cylinders to move in straight lines, keeping the piston true in the cylinder, while the walking beam end moved through an arc, somewhat analogous to a crosshead in later steam engines.

Base units
Derived units
with special names
Other accepted units
See also

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