Horsepower

Horsepower is a non-SI unit of power used chiefly to describe engine and motor output. Its value depends on the definition, including mechanical, metric, and electrical horsepower.

Power (physics): Power is the rate at which work is done or energy is transferred. Horsepower measures power, so its meaning begins with the rate of energy transfer.
Mechanical horsepower: Mechanical horsepower is a power unit equal to 550 foot-pounds per second, or about 745.7 watts. This traditional definition anchors many engine ratings in the United States.
Engine power: Engine power is the rate at which an engine delivers mechanical energy. Horsepower is a common unit for reporting an engine's output.
James Watt: James Watt was a Scottish inventor and engineer whose improvements to the steam engine transformed industrial power. Watt popularized horsepower as a way to compare steam engines with horse work.
Watt: The watt is the SI unit of power, equal to one joule per second. Watts provide the SI reference used to compare horsepower definitions.
Metric horsepower: Metric horsepower is a power unit equal to 75 kilogram-force metres per second, or about 735.5 watts. Its conversion differs from mechanical horsepower despite the shared name.
Brake horsepower: Brake horsepower is engine power measured at the output shaft using a dynamometer, before drivetrain losses. It distinguishes measured engine output from power reaching a vehicle's wheels.
Steam engine: A steam engine converts the thermal energy of pressurized steam into mechanical work. Horsepower helped communicate the useful output of Watt's improved engines.
Joule: The joule is the SI unit of energy and work. Horsepower describes how quickly energy measured in joules is transferred.
Electrical horsepower: Electrical horsepower is a power unit equal to exactly 746 watts. Electrical equipment ratings use this definition, close to but distinct from mechanical horsepower.

Linked from 9 pages