Watt
The watt is the unit of power or radiant flux in the International System of Units, equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named in honor of James Watt, an 18th-century Scottish inventor, mechanical engineer, and chemist who improved the Newcomen engine with his own steam engine in 1776, which became 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, the rate at which work is done is one watt.In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere flows across an electrical potential difference of one volt, meaning the watt is equivalent to the volt-ampere.
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.
Examples
- A person having a mass of 100 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.
- 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.
History
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. 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 in 1948, the international watt was redefined from practical units to absolute units. Concretely, this meant that 1 watt was 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 as the unit of power.
Multiples
;Attowatt: The sound intensity in water corresponding to the international standard reference sound pressure of 1 μPa is approximately 0.65 aW/m2.;Femtowatt: 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. 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.
;Picowatt: 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.
;Nanowatt: Powers measured in nanowatts are also typically used in reference to radio and radar receivers.
;Microwatt: Powers measured in microwatts are typically stated in medical instrumentation systems such as the electroencephalograph and the electrocardiograph, in a wide variety of scientific and engineering instruments, and in reference to radio and radar receivers. Compact solar cells for devices such as calculators and watches are typically measured in microwatts.
;Milliwatt: A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid uses less than one milliwatt. Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.
;Watt: PC power supply units are typically specified in watts; modern graphics cards usually have TDPs of a few hundred watts.
;Kilowatt
;Megawatt: 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, while some produce much more. The Eurostar e300, for example, uses more than, while heavy diesel–electric locomotives typically produce and use. U.S. nuclear power plants have net summer capacities between about. The earliest citing of the megawatt in the Oxford English Dictionary is a reference in the 1900 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.
;Gigawatt: A gigawatt is typical annually averaged power consumption for a city of 1.8 million habitants, and is the output of a large power station. The gigawatt is thus used for large power plants, power grids, and, increasingly, large data centers. For example, by the end of 2010, power shortages in China's Shanxi province were expected to increase to 5–6 GW and the installation capacity of wind power in Germany was 25.8 GW. The largest unit of the Belgian Doel Nuclear Power Station has a peak output of 1.04 GW. HVDC converters have been built with power ratings of up to 2 GW.
;Terawatt: The primary energy used by humans worldwide was about 160,000 terawatt-hours in 2019, corresponding to an average continuous power consumption of 18 TW that year. Earth itself emits 47±2 TW, 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 TW, but these strikes only last for 30 microseconds.
;Petawatt: 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's Nova laser, which achieved a power output of 1.25 PW by a process called chirped pulse amplification. The duration of the pulse was roughly 0.5 ps, giving a total energy of 600 J. Another example is the Laser for Fast Ignition Experiments at the Institute of Laser Engineering, Osaka University, which achieved a power output of 2 PW for a duration of approximately 1 ps. Based on the average total solar irradiance of 1.361 kW/m2, the total power of sunlight striking Earth's atmosphere is estimated at 174 PW. The planet's average rate of global warming, measured as Earth's energy imbalance, reached about 0.5 PW by 2019.
;Yottawatt: The power output of the Sun is 382.8 YW, about 2 billion times the power estimated to reach Earth's atmosphere.
Conventions in the electric power industry
In the electric power industry, megawatt electrical refers by convention to the electric power produced by a generator, while megawatt thermal or thermal megawatt refers to thermal power produced by the plant and megawatt ''mechanical to mechanical power. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2,109 MWt, which creates steam to drive a turbine, which generates 648 MWe. Other SI prefixes are sometimes used, for example gigawatt electrical''. 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 and so these unit symbols 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 per second for delivered heating power in a combined heat and power station such as Avedøre Power Station.When describing alternating current 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.