In the 18th century, the Scottish engineer James Watt introduced this unit , called the Draft Horse, a horse used for heavy work to compare the power of his steam engine . Although there have been many definitions of horsepower since then, only two are currently used. The Americans use the Imperial system , which has one horsepower = 746 watts and according to British measurements it is 635.5 watts. Initially, it was only used to test the power of steam engines, but gradually the power of all types of instruments, including piston engines , turbines , and electric motors, was measured with this horsepower. Later it was seen that some countries were using watts as a unit of power and some were using horsepower. In this case, the European Union announced in 2010 that no one in Europe would use horsepower , everyone would use watts . If anyone does, then watts and horsepower are two.