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# Horsepower

This article has been reviewed by the following Topic Editor: Tom Lawrence

Horsepower is the unit of power in the English system of measurement. The term horsepower was coined by James Watt (1736-1819), the Scottish inventor and mechanical engineer renowned for his improvements of the steam engine.

## Definition

One horsepower (hp) is equivalent to 0.7457 kW in standard SI units. A healthy human can sustain about 0.1 horsepower, a car can generate several hundred horsepower, while a steam turbine in an electric power plant can produce more than 1.5 million horsepower.

Horsepower-hour is a unit of energy or work equal to the work done by the applied power of one horsepower over the period of one hour. The corresponding standard SI unit of energy is the joule. One joule = 3.73·10-7horsepower-hour.

## History

The term horsepower was coined by James Watt (1736-1819), the Scottish inventor and mechanical engineer renowned for his improvements of the steam engine. In the early 1780s, Watt and his partner Matthew Boulton set out to sell their steam engines to the breweries of London, calculating that they would be likely customers because brewing was such an energy-intensive process. In order to convince the breweries of the advantages of the steam engine, Watt needed a method to compare their capabilities relative to horses, the power source they were seeking to replace. The typical brewery horse, attached to a mill that ground the mash for making beer, walked in an endless circle with a 24-foot diameter, pulled with a force of 180 pounds, and traveled at a speed of 180.96 feet per minute. Watt multiplied the speed times the force and came up with 32,580 ft-lbs/minute. That was rounded off to 33,000 ft-lbs/minute, the figure used today.

A healthy human can sustain about 0.1 horsepower, a car can generate several hundred horsepower, while a steam turbine in an electric power plant can produce more than 1.5 million horsepower.

## Conversions

from / to Joule kilowatt-hour electronvolt calorie Btu horsepower
hour
foot-pound dyne-
centimeter
therm
1 Joule 1 0.278 · 10?3 6.241 · 1018 0.2388 9.4782 · 10?4 3.7251 · 10-7 0.7376 1 · 107 9.4782 · 10?9
1 kilowatt-hour 3.6 · 106 1000 22.5 · 1024 8.5985 · 105 3.4121 · 103 1.3410 2.6552 · 106 3.6 · 1013 0.0341
1 electronvolt 0.1602 · 10?18 44.5 · 10?27 1 3.8267 · 10?20 1.5186 · 10?22 5.9682 · 10?26 1.1817 · 10?19 1.6022 · 10?12 1.5186 · 10?27
1 calorie 4.1868 1.163 · 10?3 0.261 · 1018 1 3.9683 · 10?3 1.56 · 10?6 3.0880 4.186 · 107 3.9683 · 10?8
1 Btu 1.055 · 103 0.293 6.585 · 1021 251.996 1 3.9302 · 10?4 778.169 1.05435 · 1010 1 · 10?5
1 horsepower-hour 2.6845 · 106 745.60 1.6755 · 1025 6.4119 · 105 2.5444 · 103 1 1.98 · 106 2.6845 · 1013 2.5444 · 10-2
1 foot-pound 1.3558 3.7662 · 10-4 8.4623 · 1018 0.3238 1.2851 · 10-3 5.0505 · 10-7 1 1.3558 · 107 1.2851 · 10-8
1 dyne-centimeter 1 · 10-7 2.7778 · 10-11 6.2415 · 1011 2.4 · 10-8 9.4782 · 10-11 3.7251 · 10-14 7.4 · 10-8 1 9.4782 · 10-16
1 therm 1.0551 · 108 2.9307 · 104 6.5851 · 1026 2.520 · 107 1 · 105 39.301 7.7817 · 107 1.0551 · 1015 1

## Citation

Cutler J. Cleveland (Lead Author);Tom Lawrence (Topic Editor) "Horsepower". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth October 8, 2007; Last revised Date October 8, 2007; Retrieved May 25, 2013 <http://www.eoearth.org/article/Horsepower>

## The Author

Cutler J. Cleveland  is Professor of Earth and Environment at Boston University, where he also is on the faculty of the Center for Energy and Environmental Studies. Professor Cleveland is Editor-in-Chief of the Encyclopedia of Energy (Elsevier, 2004), winner of an American Library Association award, the Dictionary of Energy (Elsevier, 2005), Handbook of Energy (Elsevier, forthcoming), and is the Founding Editor-in-Chief of the Encyclopedia of Earth.  He is the recipient of the Adelma ... (Full Bio)