HP to kW Converter (and kW to HP)

Convert horsepower to kilowatts and back — with all three definitions kept apart, because mechanical, metric and electrical horsepower differ by 1.4% and a motor nameplate means one of them.

The three horsepowers

Common motor sizes in horsepower

Standard IEC motor ratings in horsepower

Three horsepowers, one shaft

James Watt’s horsepower was 33,000 foot-pounds per minute, and that definition survives as mechanical horsepower: 550 foot-pounds-force per second. Because the foot, the pound and standard gravity all have exact SI definitions, so does the horsepower — 550 × 0.3048 m × 4.448221615 N/s, which is 745.6999 W. NIST’s conversion tables print it as 745.6999 W.

Metric horsepower came from restating the same idea in metric units: 75 kilogram-force metres per second, which is 75 × 9.80665 = 735.49875 W exactly. It is written PS in Germany, CV in France and Spain, pk in the Netherlands, hk in Scandinavia, and simply “HP” on a great many Indian and Gulf nameplates. It is 1.4% smaller than the mechanical one.

Electrical horsepower is a convenience: exactly 746 W, a rounding of the mechanical figure used for motor ratings in the United States. It is 0.04% above mechanical horsepower, which nobody has ever been troubled by.

Why it matters on a nameplate. A motor sold as 7.5 kW is 10.06 mechanical horsepower but 10.20 metric. Round either to “10 HP” and you have lost 2% one way or the other — which is the same order as the difference between two adjacent IE efficiency classes, and more than the tolerance on the rating itself. In the IEC world the way out is simple: the rating is in kilowatts, and any HP figure beside it is decoration. In North America the rating is in horsepower and it is the mechanical (or electrical) one. Where the two worlds meet — an Indian pump catalogue quoting HP, an American compressor quoting kW — check which definition was used by dividing the two numbers on the plate. If kW ÷ HP comes to about 0.746 it is mechanical; about 0.7355 and it is metric.

One thing all three share: a motor’s horsepower rating is output at the shaft, not input from the supply. A 10 HP motor at 90% efficiency draws about 8.3 kW. Use the motor full-load current calculator to turn a nameplate into amps, and the motor power, torque and speed calculator to turn it into torque at a given speed. For pumps, where HP ratings are most common and most misleading, the pump motor power calculator works from the flow and head the pump actually has to deliver.

Frequently asked questions

Related calculators

References

1. National Institute of Standards and Technology. Guide for the Use of the International System of Units (SI), Special Publication 811, Appendix B.8, factors for units listed alphabetically: horsepower (550 ft·lbf/s) = 7.456 999 × 10² W, horsepower (metric) = 7.354 988 × 10² W, horsepower (electric) = 7.46 × 10² W, horsepower (boiler) = 9.809 50 × 10³ W.
2. IEC 60034-30-1 efficiency classes for line-operated AC motors (IE1 standard, IE2 high, IE3 premium, IE4 super-premium), as tabulated in ABB technical note 9AKK107319 EN 05-2018. At 50 Hz and 4 poles the nominal efficiencies are 79.6 / 82.5 / 85.7% at 0.75 kW, 85.5 / 87.7 / 90.4% at 3 kW, 88.7 / 90.4 / 92.6% at 7.5 kW and 91.6 / 93.0 / 94.5% at 22 kW for IE2 / IE3 / IE4.
3. ABB. Softstarter Handbook, publication 1SFC132060M0201. Direct-on-line starting current “Usually between 6-8 times the rated current, but it can be more than 10 times the rated current”; star-delta: “The resulting current when Y-connected will be 1/3 of the current when delta connected” and the torque “ending up being 33% of the torque available when delta connected”; n = 2 × f × 60 / p; s = (n₁ − n)/n₁; T_n = 9550 × P_r/n_r.