Kilowatts and Horsepower (kW, hp and PS)

Power is the rate of doing work or using energy. The SI unit is the watt (W), one joule per second. A kilowatt (kW) is 1000 W.

Horsepower is an older unit from the age of steam engines. Several different horsepower units exist, and the two most common ones in vehicle specifications are not the same size.

Mechanical horsepower, also called imperial horsepower, is defined as exactly 550 foot-pounds-force per second. That works out to about 745.69987 W, or 0.7457 kW.

This is the horsepower used in the United States and the United Kingdom, where it is often written hp or bhp (brake horsepower, which refers to how the power is measured, not to a different unit size).

Metric horsepower is defined as exactly 75 kilogram-force meters per second, which is exactly 735.49875 W. It is written PS (from the German Pferdestärke), CV in French, Italian and Spanish, and pk in Dutch. It is slightly smaller than mechanical horsepower: 1 PS ≈ 0.98632 hp.

• Kilowatts to hp: multiply by about 1.341022. Example: 100 kW is 134.1 hp.
• Hp to kilowatts: multiply by about 0.7456999. Example: 300 hp is 223.71 kW.
• Kilowatts to PS: multiply by about 1.359622. Example: 100 kW is 135.96 PS.
• PS to kilowatts: multiply by 0.73549875. Example: 150 PS is 110.325 kW.

For a quick estimate, 1 kW is about 1.34 hp and 1 hp is about 0.75 kW.

The same engine can be listed with three different numbers depending on the market. European documents give power in kW, and brochures often add PS next to it. US specifications use mechanical hp. Because PS is smaller than hp, the PS figure is always a little higher than the hp figure for the same engine.

For example, an engine rated at 110 kW is about 149.6 PS or 147.5 hp. None of these numbers is wrong; they are the same power in different units.

Ratings can also differ because of the test method, such as where the power is measured and under which conditions. A unit conversion cannot correct for that, so compare figures from the same standard when you can.

Electric motors and electric vehicles are usually rated in kilowatts, because kW is the unit used for electrical power in general. Battery capacity is given in kilowatt-hours (kWh), which is energy, not power: a 1 kW load running for one hour uses 1 kWh.

Industrial motors are commonly rated in kW in most of the world and in hp in the United States. A 7.5 kW motor is about 10.1 hp, and a 10 hp motor is about 7.46 kW.