Horsepower refers to a unit of power that is equal to 550 foot-pounds (745.7 watts). Horsepower is the power of an engine that is measured. Horsepower is defined as work done over time. Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The SI unit of horsepower is Watt. Horsepower was first used by James Watt, who employed it to compare the power of steam engines with that of horses.
Rotating Horsepower (HP) = T * (N / 5252) Watt
T - Torque
N - Speed