Convert mechanical horsepower to watt

There is more than one horsepower. The metric horsepower (PS in German, ch in French, CV in Spanish and Italian, pk in Dutch) is defined as 75 kgf·m/s = 735.5 W. The mechanical or imperial horsepower used in the UK and US is 745.7 W, about 1.4 % more, and 'bhp' (brake horsepower) is that unit measured at the engine's output shaft. The electrical horsepower rounds it to 746 W for motor ratings, and the boiler horsepower, 9.81 kW, rates steam boilers and has nothing to do with engines. The legal unit in the EU is the kilowatt: 100 kW = 136 PS = 134 hp.

A watt is a joule per second, a rate, not an amount of energy: a 2 kW kettle running for half an hour uses 1 kWh. An LED bulb draws 5–10 W, a laptop 50, a resting human gives off about 100, a kettle 2–3 kW, a home EV charger 7–11 kW and a motorway charger 150–350 kW. A car engine of 100 kW is 136 PS or 134 hp. A modern onshore wind turbine is rated at about 5 MW, a nuclear reactor 1–1.6 GW, and Germany draws some 80 GW on a winter evening.