Even more interesting is how the definition came to be. It was originated by James Watt, (1736-1819) who is credited with making the steam engine an industrial practicality and the man whose name has been immortalized by the definition of Watt as a unit of power. The next time you complain about the landlord using only 20 watt light bulbs in the hall, you are honoring the same man.
To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that grinded corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known. Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today.
Put into perspective, a healthy human can sustain about 0.1 horsepower. Most observers familiar with horses and their capabilities estimate that Watt was a bit optimistic; few horses could maintain that effort for long.
Although the standard for rating horsepower has been available for over 200 years, clever car manufacturers have found ways to change the ratings of their engines to
suit their needs. During the famous horsepower wars of the 1960s, manufacturers could get higher figures by testing without auxiliary items such as alternators or even water pumps. High ratings backfired when insurance companies noticed them and started to charge more for what they saw as a higher risk. Manufacturers sometimes responded by listing lower horsepower figures, forcing enthusiasts to look at the magazine test reports to determine what was going on. In the early seventies the SAE (Society of Automotive Engineers) stepped in with standardized test procedures and the fiqures were more consistent.
You may occasionally come across the term "Brake Horsepower". This refers to the measurement method where then engine, at full throttle, is held to a specific RPM and the output is measured.
Between 1922 and 1947, the Royal Automobile Club used a horsepower rating that was the basis for an automobile tax. The horsepower of an engine was determined by multiplying the square of the cylinder diameter in inches by the number of cylinders and then dividing that figure by 2.5. Using this dubious method, What we know of as a 385 horsepower motor found in the 2001 Z06 Corvette would be rated at only 48.67 hp!
There is a metric horsepower rating, although it is rarely used. The two methods are close, with one SAE horsepower equal to 1.0138697 metric horsepower.
One mechanical horsepower also equals 745.699 watts or .746 kW (kilowatts) of electrical horsepower. This means that if you really want to confuse people, you could complain about the 0.0268 horsepower light bulb your landlord has in the hallway as opposed to the mundane 20 watt measurement.
For more information we recommend http://en.wikipedia.org/wiki/