Horsepower (hp) is the unit of measurement for power (the time – rate of change of work done). There are many different standards and kinds of horsepower. The most commonly used is — especially for electrical power — 1 hp = 746 watts. This term was adopted in the late 18th century by a Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was then later expanded to include the output power of other various types of piston engines, as well as turbines, electric motors and other machinery.
The measure of an engine's horsepower before the loss in power caused by the gearbox and drive terrain Brake horsepower (bhp). In Europe, the DIN standard tested the engine fitted with all ancillaries and exhaust system as used in the vehicles. The American SAE system tests without water pump, alternator and other auxiliary components such as muffled exhaust system, power steering pump, etc. so the figures are larger than the European figures for the same engine. Times before, Brake were referred to as the device which were used to load an engine and hold it at a desired rotational speed. During testing, the output torque and rotational speed were measured to make out the brake horsepower. Horsepower was originally measured and calculated with the help of the indicator diagram (a James Watt’s invention of the late 18th century) and later by the means of a de Prony brake which was connected to the engine's output shaft.
In the United States, the term bhp was stopped using during 1971–72, because an automakers began to quote power in terms of SAE net horsepower in accord with the SAE standard – J1349. Like SAE gross and other brake horsepower protocols, SAE Net horsepower is measured at the engine's crankshaft, and so it does not account for the transmission losses. Although, the SAE net power testing protocol calls for standard production – type belt – driven accessories, air cleaner, emission controls, exhaust system, and other power – consuming accessories. This produces ratings in closer alignment with the power made by the engine as it is actually configured and sold. Most recently, an electrical brake dynamometer was used instead of a De Prony brake. Although, the delivered output was less than that obtainable at the engine's crankshaft, a chassis dynamometer gave an indication of the engine's real world horsepower after losses in the drive train and gearbox, to the driving wheels.