Convert power

About the units

• Watts (W) – The watt is the SI coherent unit of power, one joule per second, and the International Committee for Weights and Measures put it in 1946 as the power that gives rise to one joule of energy in one second. It is a legal unit throughout the EU and the marking on lamps, chargers, heat pumps and household appliances. Two traps. The watt is power and the watt-hour energy, how fast against how much. And the name is written in lower case while the symbol is a capital W.
• Kilowatts (kW) – A kilowatt is 1,000 watts and the legal unit of power for vehicles in the EU: the type-approval information document requires engine power in kilowatts. Chargers, heat pumps and solar installations are rated in kilowatts too. One kilowatt is 1.341 mechanical horsepower, 1.360 metric, and about 3,412 Btu per hour. Two traps. The kilowatt is power and the kilowatt-hour energy – one kilowatt for one hour is exactly one kilowatt-hour. And a 150 kW car is a little over 200 horsepower, depending on the variant.
• Megawatts (MW) – A megawatt is a million watts, since mega means 10⁶, and a legal unit throughout the EU. It measures power stations, grids and large consumers: the Swedish reactor Forsmark 3 is rated at 1,172 megawatts. The trap is power against energy. Running at full power for a whole 365-day year, Forsmark 3 would produce 10.3 terawatt-hours, whereas in 2025 it produced about 8.8. The megawatt says how much is delivered at any moment, the megawatt-hour how much has been delivered.
• Horsepower (mechanical) (hp) – Horsepower comes in several variants. This conversion uses the mechanical one, defined as 550 foot pounds-force per second, which makes it 745.69987158227 watts. Metric horsepower – Swedish hk, German PS – is 75 kilogram-force metres per second, 735.49875 watts, and electric horsepower is exactly 746 watts. Horsepower is absent from the EU units directive and may appear only as a supplementary indication beside the kilowatt figure. The trap is therefore the number itself: 150 PS is 110.3 kW, while 150 hp is 111.9.
• Metric horsepower (PS) – Metric horsepower is 75 kilogram-force metres per second, exactly 735.49875 watts, and it is what German PS and Swedish hk mean. The first EEC units directive, in 1971, listed PS among the units to disappear by 31 December 1977 at the latest, and EU type approval now requires engine power in kilowatts, yet the metric horsepower lives on in the motoring press. The big trap is mechanical horsepower, hp, at 745.7 watts: 100 PS is 73.5 kilowatts but 100 hp is 74.6, a gap of 1.4%.
• BTU per hour (BTU/h) – The Btu per hour is a power: energy divided by time. With the international table Btu, one Btu per hour is 0.29307107017222 watts, so one kilowatt is about 3,412 Btu per hour. It is the American unit for heating and cooling capacity; US labelling rules require pool heater capacity in Btu per hour, rounded to the nearest thousand. In the EU it is not a legal unit. The trap is that the trade writes “12,000 BTU” when it means per hour.