Horsepower to Watts Calculator

How it works

Mechanical watts = horsepower × 745.6999. Kilowatts = watts ÷ 1,000. NIST lists metric horsepower as 735.4988 W, electric horsepower as 746 W and boiler horsepower as 9,809.50 W, so confirm which “hp” a source uses. For a motor, approximate electrical input = mechanical output ÷ efficiency, but voltage, current, phase, power factor, load and manufacturer data are required for a responsible electrical estimate.

Worked example: 5 mechanical horsepower

5 hp × 745.6999 W/hp = 3,728.4995 W, displayed as 3,728.50 W. Divide by 1,000 to get 3.7285 kW. That is the mechanical power represented by the unit conversion.

It is not a prediction of wall electricity. At a hypothetical 90% efficiency and rated output, input real power would be about 3.7285 ÷ 0.90 = 4.14 kW, before separately determining current from system voltage, phase and power factor.

Mechanical, metric, electric and boiler hp differ

NIST lists several units that share the horsepower name. Mechanical horsepower is 745.6999 W; metric horsepower is 735.4988 W; electric horsepower is 746 W; boiler horsepower is 9,809.50 W. Boiler horsepower is a heat-output convention and is not interchangeable with motor hp.

This calculator intentionally uses mechanical horsepower. If a European equipment document uses PS, CV or metric hp, convert with the metric factor instead.

Motor nameplate hp is useful output, not electricity use

DOE defines motor efficiency as useful output divided by input. Electrical input must therefore exceed mechanical output when a motor is delivering rated load. Actual input also changes with load and motor efficiency at that load.

Do not estimate billable kWh by multiplying nameplate hp × 0.746 × all elapsed hours unless the motor truly delivers rated output continuously. Use measured real kW or manufacturer performance data for the operating point.

Do not size circuits or generators from hp alone

Motors can draw substantial starting current, and nameplates include voltage, phase, frequency, rated current, service factor and other requirements. Generator starting capability, conductor and protection design cannot be derived from the horsepower-to-watts unit conversion alone.

Use manufacturer starting data, nameplate requirements, applicable electrical code and qualified design. A clamp reading must also be obtained with suitable equipment and safe access procedures.

Sources used for assumptions: NIST Guide to the SI — horsepower-to-watt conversion factors; U.S. Department of Energy — motor selection guide and efficiency definitions; U.S. Energy Information Administration — electricity glossary for power and watts.

Frequently asked questions