HP to kW Converter

About the HP to kW Converter

Convert horsepower to kilowatts and kilowatts to horsepower for all three horsepowers in use: mechanical hp (550 ft·lbf/s = 745.7 W — the hp and bhp of US and British spec sheets), metric hp (PS, CV, ch or pk: 75 kgf·m/s = 735.5 W) and electric hp (746 W, for electric motors). Pick which one you have, type in either box, and the other two appear under the result: 100 hp is 74.57 kW, or 101.39 PS.

The tables list motor and pump sizes from 0.5 to 10 hp in kW, car and motorcycle power from 50 to 1,000 hp or PS, and kW from 1 to 1,000 in hp and PS. Electricity a motor draws turns a motor’s rated output into the power it takes from the supply, from its efficiency.

Common uses

• Pump and motor nameplates in hp and kW.
• Car and motorcycle spec sheets in hp, bhp, PS and kW.
• Generators, compressors and outboard motors rated in hp.
• Estimating the electricity a motor or pump uses per hour.

Three horsepowers

NIST SP 811, Appendix B.8 lists them all in watts:

• Mechanical horsepower (hp): 550 foot-pounds-force per second = 745.69987 W. kW = hp × 0.7457, hp = kW × 1.341.
• Metric horsepower (PS, from the German Pferdestärke; CV, ch and pk in other languages): 75 kilogram-force metres per second = 735.49875 W exactly. kW = PS × 0.7355, PS = kW × 1.3596.
• Electric horsepower: exactly 746 W, used for electric motors.

So 1 hp = 1.0139 PS and 1 PS = 0.9863 hp: 100 PS is 98.6 hp, and the gap grows with the number.

bhp, whp and PS on spec sheets

• bhp (brake horsepower) is the engine’s power at its output shaft, measured on an engine dynamometer — the “brake” is the device that loads the engine. It is in mechanical horsepower.
• whp (wheel horsepower) is measured at the driven wheels on a chassis dynamometer, after the gearbox, final drive and tyres have taken their share, so it is lower than bhp. The loss differs from car to car, so no fixed factor turns one into the other.
• PS is the metric horsepower many European and Asian spec sheets use.

In the EU the legal unit of power is the watt, so spec sheets give kilowatts: Council Directive 80/181/EEC lets other units appear only as supplementary indications beside it, in characters no larger, which is why brochures print “110 kW (150 PS)”. In the US, engine ratings follow test codes such as SAE J1349, the code for an engine’s net power as installed in the vehicle.

Motors and pumps

The hp or kW on a motor or pump is the power it delivers at its shaft. It takes more than that from the supply, because no motor is perfectly efficient: input power = output ÷ efficiency. A 1 hp motor (0.7457 kW) at 75 % efficiency draws about 0.99 kW, so it uses about 1 kWh of electricity for each hour it runs at full load. Nameplates often also round to the standard metric ratings, labelling a 1 hp motor 0.75 kW and a 10 hp motor 7.5 kW.

Limitations

• The conversions are exact; only the display is rounded (10 significant figures, or choose a Precision).
• Power figures measured under different test standards (gross or net, SAE, DIN or ECE) can differ by more than any unit conversion; this converter changes only the unit.
• The input-power estimate assumes the motor runs at its rated output. At part load it draws less, and for the current in amps you also need the voltage and the power factor — use the Watts to Amps Calculator.
• Engine displacement (cc) and torque (N·m) are not power and are not converted here.

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