Horsepower (hp) is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts. Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the output of piston engines, turbines, electric motors, and other machinery. The definition of the unit varied between 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 only permitted as supplementary unit.
Units called "horsepower" have differing definitions:
The development of the steam engine provided a reason to compare the output of horses with that of 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 idea was later used 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 × 2π × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. So:
I was born in a hurry but there wasn't any place to go
Nowhere fast seemed better than nowhere slow
I never really got the hang of hangin' around
When you get down to it
I guess, I always knew it
What it is that makes my world go 'round
It's a matter of horsepower
For 8 seconds on miles an hour
That's the way I am and it's a fact I can't ignore
Yeah, big four legged, fuel injected
Runnin' wild, radar detected
Man can always stand a little bit more
Horsepower
Well, the view looks better from ahead than it looks behind
I've got a need for speed and I don't mean the drugs-store kind
Some people like a life that barely moves at all
I say more power to 'em
Then I fly right through 'em
'Til it slows down to a crawl
And then I think about horsepower
For 8 seconds on miles an hour
That's the way I am and it's a fact I can't ignore
Yeah, big four legged, fuel injected
Runnin' wild, radar detected
Man can always stand a little bit more
Horsepower
Then I think about horsepower
For 8 seconds on miles an hour
That's the way I am and it's a fact I can't ignore
Yeah, big four legged, fuel injected
Runnin' wild, radar detected
Man can always stand a little bit more