Even more interesting is how the definition came to be. It was originated by James Watt, (1736-1819) the inventor of the steam engine 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.

So, they could test it by making a car weigh 33,000 lbs. and see how far it could get in a minute. That would be the simple way to test.

{which I don`t understand}, the horsepower of the vehicle can be calculated.

In case you are wondering if they test every car, well they don’t, usually they just test the model car and assume every car built like it has the same HP.

ok just kidding on all that ^ but really, it use to have something to do with horses. Maybe so that people who still used horses would have something to compare it to.