Watts to Horsepower Converter
Three horsepowers, kept apart, because a figure quoted in PS isn't the same as one quoted in hp.
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There is no single horsepower. Mechanical hp is 745.7 W, metric hp (PS) is 735.5 W, and electrical hp is exactly 746 W. A car advertised at 200 PS in Germany is about 197 hp in Britain, and neither figure is wrong. Most converters pick one and don't say which.
Reference only. Motor sizing, circuit protection and conductor selection must follow the electrical code that applies where you are, and should be verified by a qualified electrician. A nameplate horsepower rating isn't the same as the power the motor draws from the supply.
The formulas
1 metric hp (PS) = 75 kgf-m/s = 735.49875 W (exact)
1 electrical hp = 746 W (exact, set by IEEE and NEMA)
All three are exact definitions, but they behave differently in decimal. Metric horsepower is 75 kgf-m/s, and since standard gravity is exactly 9.80665 m/s², that works out to exactly 735.49875 W. Electrical horsepower was set at exactly 746 W by IEEE and NEMA as a clean round figure for motor ratings. Mechanical horsepower is exactly 550 ft-lbf/s, but that expansion doesn't terminate in decimal, so the 745.699872 shown above is rounded.
The watt itself is exact, being one joule per second.
Conversion table
| Watts | Mechanical hp | Metric hp (PS) | Electrical hp |
|---|---|---|---|
| 100 W | 0.1341 | 0.1360 | 0.1340 |
| 250 W | 0.3353 | 0.3399 | 0.3351 |
| 500 W | 0.6705 | 0.6798 | 0.6702 |
| 746 W | 1.0004 | 1.0143 | 1.0000 |
| 1,000 W | 1.3410 | 1.3596 | 1.3405 |
| 2,000 W | 2.6820 | 2.7192 | 2.6810 |
| 5,000 W | 6.7051 | 6.7981 | 6.7024 |
| 10,000 W | 13.4102 | 13.5962 | 13.4048 |
| 50,000 W | 67.0511 | 67.9811 | 67.0241 |
| 100,000 W | 134.1022 | 135.9622 | 134.0483 |
Where the difference shows up
Vehicle marketing is the main place. A specification quoted in PS, CV or pk is metric horsepower and reads about 1.4 percent higher than the same engine quoted in mechanical hp. Over a 300 hp engine that is roughly four horsepower of pure unit choice.
Motor nameplates are the other. Electrical horsepower at exactly 746 watts exists precisely so that motor ratings have a clean relationship to electrical power, which the watts to amps converter handles on the current side.
Common questions
How many watts is 1 horsepower?
It depends which horsepower. Mechanical horsepower is 550 foot-pounds-force per second, about 745.7 watts. Metric horsepower is 75 kilogram-force meters per second, which is exactly 735.49875 watts. Electrical horsepower was set at exactly 746 watts by IEEE and NEMA. They differ by up to about 1.4 percent.
Which horsepower do car manufacturers use?
European makers usually quote PS, the metric horsepower of 735.5 watts. American and British figures are normally mechanical horsepower at 745.7 watts. A car quoted at 200 PS is about 197 mechanical hp, not 200.
Is horsepower an SI unit?
No. The watt is the SI unit of power. Horsepower is a customary unit that survives in automotive and some machinery contexts, and it has never had a single agreed definition.
What is 1000 watts in horsepower?
1,000 watts is 1.341 mechanical horsepower, or 1.360 metric horsepower. The two differ because mechanical horsepower is 550 foot-pounds-force per second, about 745.7 watts, while metric horsepower is 75 kilogram-force meters per second, exactly 735.49875 watts.
Related converters and guides
Sources
- NIST Guide to the SI, Appendix B: Conversion Factors. National Institute of Standards and Technology. Source for every conversion factor on this page, and for which factors are exact rather than rounded.