Horsepower can refer generally to the power output of an engine, or it can refer to one of several specific measurement units.

Brake horsepower (bhp) has long been the standard UK/US unit of engine power. 1bhp = 745.7 watts.

Pferdestärke (PS) is 'metric horsepower', defined using the SI system but very nearly the same as bhp. 1PS = 735.5 watts.

The RAC horsepower rating is very different: it was a taxation system which rated engines using a calculation which was intended to reflect engine power, though in practice it was too simplistic to be very accurate.