Power Conversions


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Watt - The watt (symbol: W) is the SI derived unit of power, equal to 1 joule (J) of energy per second. It measures a rate of energy conversion. A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typical automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts, with LED lights typically between 1 and 5 watts. In terms of mechanical energy, one watt is the rate at which work is done when an object is moved at a speed of one meter per second against a force of one newton. 1W = 1Js-1 = 1kgm2s-3 = 1Nms-1. By the definitions of electric potential (volt) and current (ampere), work is done at a rate of one watt when one ampere flows through a potential difference of one volt. 1W=1Vx1A

The watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th General Conference on Weights and Measures in 1960 as the unit of power incorporated in the International System of Units (or "SI").

This SI unit is named after James Watt. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (W). When an SI unit is spelled out in English, it should always begin with a lowercase letter (watt), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

Kilowatt - The kilowatt (symbol: kW), equal to one thousand watts, is typically used to state the power output of engines and the power consumption of tools and machines. A kilowatt is approximately equivalent to 1.34 horsepower. An electric heater with one heating element might use 1 kilowatt. The average annual power consumption of a household in the United States is about 8,900 kilowatt-hours.

Megawatt - The megawatt (symbol: MW) is equal to one million (106) watts. Many things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: lightning strikes, large electric motors, naval craft (such as aircraft carriers and submarines), engineering hardware, and some scientific research equipment (such as supercolliders and large lasers). A large residential or commercial building may consume several megawatts in electric power and heating energy.

The productive capacity of electrical generators operated by utility companies is often measured in MW. Modern high-powered diesel-electric railroad locomotives typically have a peak power output of 3 to 5 MW, whereas U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW. The earliest citing for "megawatt" in the Oxford English Dictionary is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November 1947, article in Science (506:2).

Horsepower - Horsepower (hp or HP or Hp) is the name of several non-SI units of power. It was originally defined to allow the output of steam engines to be measured and compared with the power output of draft horses. The horsepower was widely adopted to measure the output of piston engines, turbines, electric motors and other machinery. Different regions adopted different definitions of the unit. Most countries now use the SI unit watt for measurement of power.

The definition of a horsepower unit is different in different applications; application outside of the context of a particular definition will be inaccurate. One mechanical horsepower of 550 foot-pounds per second is equivalent to 745.7 watts. A metric horsepower of 75 kgf-m per second is equivalent to 735.499 watts. A boiler horsepower is used for rating steam boilers and is equivalent to 34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, or 9809.5 watts. One horsepower for rating electric motors is equal to 746 watts .A Pferdestarke is a name for a group of similar power measurements used in Germany around the end of the 19th century, all of about one metric horsepower in size. An ALAM or SAE horsepower is not a unit of power but instead is a measure of internal combustion engine displacement, and an RAC horsepower or tax horsepower is a differently defined unit of displacement.

Where units of horsepower are used for marketing consumer products, often measurement methods are designed by advertisers to maximize the size of the number produced for any product, even if this may not reflect realistic capacity of the product to do work when used in normal conditions.

The development of the steam engine provided a reason to equate the output of horses with the engines that could replace them. In 1702, Thomas Savery wrote in The Miner's Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work..." The term "horsepower" was coined later by James Watt to help market his improved steam engine. He had previously agreed to take royalties of one third of the savings in coal from the older Newcomen steam engines. This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius, therefore the horse travelled 2.4 x 2pi x 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds (assuming that the measurements of mass were equivalent to measurements of force in pounds-force, which were not well-defined units at the time).

Metric Horsepower - Metric horsepower began in Germany in the 19th century and became popular across Europe and Asia. The various units used to indicate this definition ("PS", "CV", "pk", and "ch") all translate to "horse power" in English, so it is common to see these values referred to as "horsepower" or "hp" in the press releases or media coverage of the German, French, Italian, and Japanese automobile companies. British manufacturers often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question.

Metric horsepower, as a rule, is defined as 0.73549875 kW, or roughly 98.6 percent of mechanical horsepower. This was a minor issue in the days when measurement systems varied widely and engines produced less power, but has become a major sticking point today. Exotic cars from Europe like the McLaren F1 and Bugatti Veyron are often quoted using the wrong definition, and their power output is sometimes even converted twice because of confusion over whether the original "horsepower" number was metric or mechanical.