Horsepower
Symbol: HP
The horsepower is not recognized in the International System of Units (SI).
It is an old unit of power. It is traced back to James Watt who was looking for a way to compare the power of horses with the power of steam engines. He calculated that the power of a horse working at a mill equals to an average power of 33000ft*lbs/min.
The German DIN-PS is defined to be the power needed to lift a mass of 75kg with a speed of 1m/s against the gravitational force of the earth. For this reason one PS equals 735.49875 W. Vice versa 1kW = 1.35962162 HP(PS).