The term horsepower came from Scottish inventor James Watt. The value for a unit of horsepower was determined after Watt made an extensive study of horses pulling coal. He originally determined that the average horse was able to lift 33,000 pounds of coal one foot in one minute. The conversion between watts and horsepower (hp) is that 1 hp = 746 W = 0.746 kW.

In the United States automobile engines are rated in horsepower while in the rest of the world kilowatts are used. In a hybrid car the power of the gasoline engine is usually measured in hp while the electric motor is measured in kW.