What is the origin of the "horsepower"? Why would anyone wish to express power in the unit of horsepower? How many watts are in one horsepower?

Horsepower unit was first time used by James Watt in 1782. The story refers that James Watt uses worked with pony to charge coal from the mines. According to that story, he have the need of a unit to measure the force from one of this animals. He founds that they can move 22.000 lbs per minute, so he (arbitrarily) increase this measure in 50% been the unit Horsepower in 33.000 lb/feet per minute.

Explanation:

This measure unit can measure "work" or "force". In the SI correspond to move up 75 Kg, to 1 meter high, in one second.

1 HP = (330 lb) x (100 feet) / 1min = 33000 lb x feet/min

Why would anyone wish to express power in HP?

Is a practical unit, because reduce the amount of digits in a specific value. Also, his use is very spread specially in mechanical applications.

1 HP = 746 W (0,746 kW).