Convert Horsepower to Watts

About Horsepower to Watts Conversion

Horsepower (hp) and watts (W) are both units of power, meaning the rate of doing work or transferring energy. The watt is the SI unit used in electrical, scientific, and most international technical documents, while horsepower remains common for engines, motors, pumps, compressors, and other machine ratings. When horsepower is converted to watts, the numerical value becomes larger because one horsepower is much larger than one watt.

Here, plain horsepower refers to mechanical horsepower. In that form, 1 horsepower = 550 foot-pounds-force per second, which is 745.69987158227022 watts. To convert horsepower to watts, multiply by 745.69987158227022. For rough work this is often rounded to 746 W per hp, but exact or close engineering work should keep more digits until the final rounding step.

Exact Relationship

The mechanical horsepower used here is built from older foot-pound-force units, while the watt is an SI unit. NIST lists horsepower in the form horsepower (550 ft lbf/s), and the modern international foot is defined as exactly 0.3048 meter.

Using the international foot and the standard pound-force definition, 1 ft lbf/s = 1.3558179483314004 W, so 550 ft lbf/s = 745.69987158227022 W. That is the basis of the conversion on this page. Published reference tables often round the factor to 745.6999 W or simply 746 W when that level of precision is sufficient.

Example Calculation

Suppose a motor is rated at 15 horsepower and you want the same power in watts. Multiply 15 by 745.69987158227022.

15 hp x 745.69987158227022 W/hp = 11185.4980737340533 W

Rounded to a practical figure, that is about 11185.5 W, or about 11.19 kW. The number becomes larger because the watt is the smaller unit.

Example for Motors and Generators

Industrial motors, pumps, and engine-driven equipment are often advertised in horsepower, while electrical systems are sized in watts or kilowatts. A 50 hp compressor corresponds to 37284.993579113511 W of mechanical output, usually stated as about 37.3 kW.

That comparison is useful when matching a motor to a variable-frequency drive, estimating shaft output against an electrical rating, or comparing a machine specification from a horsepower-based catalog with an SI-based engineering document. The converted watt figure describes power, but the final electrical demand may still be higher once efficiency losses and operating conditions are considered.

Common Benchmarks

These reference values are useful for quick comparisons between horsepower ratings and SI power values.

Cautions

Do not assume every horsepower is the same unit. Mechanical horsepower, metric horsepower, electric horsepower, boiler horsepower, and other variants all differ. This page uses mechanical horsepower, the site's default horsepower entry, not metric or boiler horsepower.

Do not confuse output power with electrical input. A machine delivering 10 hp of shaft power may require more than 7457 W from the supply once motor efficiency, drive losses, or other system losses are included.

Also distinguish power from energy. Watts and horsepower describe a rate, while watt-hours and kilowatt-hours describe energy used over time.

Reference

• NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of science
• NIST Guide to the SI, Appendix B: Conversion Factors
• BIPM, The International System of Units (SI)