Power units explained
Power is a rate: how fast energy is moved or used. The confusion in this category is not the maths but the horse — there are two different horsepowers, and the same engine is sold under both.
Watt W
Base unit — every factor on this page is measured against it
The SI unit: one joule per second. Named for James Watt, which is a small irony given that he invented the horsepower this unit eventually displaced. A person at rest gives off about 100 W as heat.
Kilowatt kW
1 kW = 1,000 W Exact
A thousand watts. The unit for electric motors, kettles, heat pumps and electric car charging. A domestic kettle is about 3 kW, a home charger 7, and a rapid charger 50 or more.
Megawatt MW
1 MW = 1,000,000 W Exact
A million watts. The scale of power stations and grid capacity. A large wind turbine is rated around 3 to 5 MW, a gas plant several hundred, a nuclear reactor around a thousand.
Mechanical horsepower hp
1 hp = 745.699872 W Exact
Mechanical horsepower: 550 foot-pounds per second, about 745.7 W. The figure comes from James Watt measuring a mill horse and then choosing a number that made his engines look good — most real horses cannot sustain it.
Metric horsepower PS
1 PS = 735.49875 W Exact
Metric horsepower, written PS in Germany, CV in France and Italy, and hk in Scandinavia. Exactly 735.49875 W, about 1.4 percent smaller than mechanical horsepower. A car sold as 150 PS in Germany is 148 hp in Britain, with nothing about the engine changed.
BTU per hour BTU/h
1 BTU/h = 0.29307107 W Exact
BTU per hour: a rate of heat movement, even when the per-hour is dropped in conversation. Air conditioning is rated this way, where 12,000 BTU/h is one ton of cooling and shifts about 3.5 kW of heat.
Foot-pound per second ft·lbf/s
1 ft·lbf/s = 1.355818 W Exact
Foot-pounds per second, the unit horsepower was originally defined against — 550 of them make one. Rarely used directly now, but it is the reason mechanical horsepower has such an untidy value in watts.