HP to Watts Calculator

James Watt coined the term "horsepower" around 1783 to market his improved steam engine. He needed a way to tell coal mine operators how many horses his engine could replace. After observing mill horses, he estimated that a strong horse could do 33,000 foot-pounds of work per minute by turning a mill wheel in a circle. That figure became the definition of one mechanical horsepower.

Whether Watt's measurement was accurate is debatable. Modern studies suggest a horse can sustain about 0.7 HP over an extended period, with brief bursts up to 15 HP. But the number stuck because it was useful, not because it was precise. Two centuries later, we still rate motors, engines, and compressors in a unit based on an 18th-century estimate of equine output.

The watt, named after the same James Watt, is the SI unit of power. One watt equals one joule per second. It is a precisely defined unit, unlike horsepower: NIST's table of conversion factors (SP 811, Appendix B.9) lists six horsepower rows, from the 550 foot-pound-force per second horsepower to boiler and water horsepower. When you see a motor rated in HP, the first question should be: which horsepower? For an electric motor it is electrical horsepower, 746 watts. Our locked rotor amps calculator takes the nameplate horsepower as printed and multiplies it by the code letter's kVA per horsepower to estimate the starting current a motor draws.

Mechanical (Imperial) Horsepower: 745.7 watts. This is the original Watt definition: 33,000 foot-pounds per minute. NIST lists it as the 550 foot-pound-force per second horsepower. Use it for engines and for shaft power stated in mechanical horsepower.

Electrical Horsepower: 746 watts. NIST lists the electric horsepower at 746 watts, and it is the rating of an electric motor. ABB's Baldor-Reliance guide to reading a NEMA motor nameplate says "For an electric motor, one horsepower is equivalent to 746 watts of electrical power and is the standard rating in the United States." NEMA's motor efficiency slides write "1 HP = 746 WATTS", and the federal motor efficiency rule, 10 CFR 431.25, converts a kilowatt rating at 1 kilowatt = 1/0.746 horsepower. This calculator uses electrical horsepower by default. The difference from mechanical HP is 0.3W per horsepower, 0.04%.

Metric Horsepower (PS): 735.5 watts. Defined as 75 kilogram-force meters per second. Common in European and Asian automotive specifications. A car rated at 200 PS (Pferdestärke, literally "horse strength") has about 197 mechanical HP. The 1.4% difference occasionally causes confusion when comparing vehicle specs across markets. If the motor drives a pump in a solar installation, our solar panel output calculator shows whether your array produces enough daily energy to run it. If you are converting a European motor rating for use in a North American installation, the metric-to-watts conversion gives you the correct power figure to work with.

Values use the mechanical horsepower definition (745.7 watts). For an electric motor's nameplate horsepower, multiply by 746; for metric HP, multiply the HP value by 735.5 instead. The kilowatt column is what you will find on European motor nameplates: a "7.5 kW" motor is the metric equivalent of a 10 HP motor.