Measure · 5 units
Power
Power is energy per time; the watt is 1 J/s. The horsepower comes from James Watt’s figure of 33 000 foot-pounds per minute for a working horse; the metric horsepower is a slightly smaller, rounded metric version.
Convert power
Then and now
Car engines in Europe were rated in PS (metric horsepower) and in the US in hp; both are now officially given in kW.
All power units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| watt W | 1 J/s | 1 W | exact | BIPM SI Brochure |
| kilowatt kW | 1000 W | 1 000 W | exact | BIPM SI Brochure |
| Imperial / US | ||||
| horsepower (mechanical) hp | 550 foot-pounds per second | 745.699 871 582 270 22 W | exact | NIST SP 811 |
| Engineering | ||||
| BTU per hour BTU/h | 1 BTU (IT) in 1 hour | 0.293 071 070 172… W | exact | NIST SP 811 |
| Gravitational metric | ||||
| metric horsepower (cheval vapeur) PS | 75 kilogram-force metres per second | 735.498 75 W | exact | NIST SP 811 |