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If you are driving a car and you are asked "how many horsepower is that vehicle?", you could find the answer by looking at the vehicle specification. 50 horsepower? 90 horsepower? Those are the kinds of figures. But if you were riding a horse and were asked the same question "how many horsepower is that vehicle?" you might be tempted to answer "1". But it's not true. In vehicle performance terms the actual horsepower of a horse is actually many times that!
Curiously, a horsepower is defined as 550 foot-pounds per second, which is about 746 watts. The idea of "horsepower" has a more glamorous sound to it, though, and it's possible to say things like "four horsepower electric kettle" instead of "3 kilowatt electric kettle".
The reason why the horsepower got defined as about three-quarters of a kilowatt is because it's based on how much work you can get a horse to do in a day. The definition of horsepower dates back to the the time when pumping of water out of mine shafts could be done by having a horse on a treadmill or by the more modern method of using an engine. Industrialists considering whether they should upgrade to steam power could be tempted by how many horses could be replaced by a steam engine and therefore how big a cost saving this represented. So, where a place previously required a team of eight horses to be kept, looked after, and be put to work in turns on different shifts, the size of steam engine required to pension them off would be eight horsepower.
Less seriously, see The Gerbil Hour
Also see Horsepower Freaks