Power Conversion Calculator — W, kW, hp and Btu/h

Background.

Power is the rate at which energy is transferred or work is done. Different industries express that rate in watts, kilowatts, horsepower, Btu per hour or kilocalories per hour. This calculator preserves one power rate while changing the unit, with the specific horsepower, calorie and Btu conventions named rather than hidden.

The watt is the coherent SI derived unit. One watt is one joule per second. One kilowatt is exactly one thousand watts and one megawatt one million. Because energy and time are both part of the definition, a power conversion must not be confused with energy: kW is a rate, while kWh is the energy accumulated when that rate lasts for hours.

Mechanical horsepower is defined here as 550 foot-pound-force per second. Using the exact international foot, pound and standard gravity gives 745.69987158227022 watts. Metric horsepower, normally labelled PS, is 75 kilogram-force metres per second, exactly 735.49875 watts. The two differ by about 1.4 percent, enough to matter when comparing vehicle or motor specifications.

Heat-rate units follow by dividing named energy units by time. One International Table Btu per hour is 1,055.05585262 joules divided by 3,600 seconds, or 0.293071070172222... watts. One thermochemical kilocalorie per hour is 4,184 joules divided by 3,600 seconds, or 1.162222222222... watts.

The page multiplies the input by watts per source unit, then divides the unrounded canonical value by watts per target unit. Every supporting output comes from the same value. Decimal.js carries the factors and each number rounds once at ten decimal places.

The worked example converts 100 kW to about 134.1022089594 mechanical hp and 135.9621617302 metric PS. Those two simultaneous figures are a useful warning: a catalogue that prints only “135 horsepower” has not supplied enough information for an exact comparison.

Signed power is allowed. In an electrical, thermal or mechanical balance, a convention may treat power entering a system as positive and leaving as negative, or the reverse. Conversion preserves that sign without choosing the convention. Zero is valid; non-finite input and unknown units are rejected.

Unit equivalence does not imply equipment equivalence. A 10 kW motor input rating does not guarantee 10 kW of shaft output because losses intervene. A heat pump can deliver more thermal kW than its electrical input through a coefficient of performance, while a combustion engine delivers less shaft power than the chemical rate in its fuel. This page applies no efficiency.

Capacity labels also need context. Btu/h on an air conditioner is thermal output, not necessarily electrical demand. Boiler horsepower is a different convention and is not one of the supported options. Electrical horsepower variants exist as well; this selector offers mechanical hp and metric PS only.

Power and torque are related through rotational speed, not interchangeable. To calculate engine power from torque and RPM, use the live horsepower calculator. To convert a known torque unit, use the newton-metre converter. This page starts only after the quantity is already power.

The displayed precision describes the conversion relationship, not measurement certainty. Nameplate tolerances, operating conditions, test standards and duty cycles remain attached to the source figure. Retain the original unit definition and significant figures when the result supports an engineering decision.

What is power conversion calculator?

A power converter changes the unit used for an energy-transfer rate. Watt is joule per second. Mechanical horsepower and metric horsepower are distinct rate definitions; Btu/h and kcal/h divide named heat-energy units by an hour. Conversion does not apply efficiency or calculate power from other physical inputs.

How to use this calculator.

1. 01Enter the signed power and choose the exact source convention.
2. 02Choose the target unit, distinguishing mechanical hp from metric PS.
3. 03Read watts or kilowatts as the canonical cross-check.
4. 04Keep input/output or generation/consumption sign conventions with the result.
5. 05Apply equipment efficiency separately when converting an input rating to useful output.

The formula.

Every unit is assigned watts per unit. Multiply by the source factor to reach watts and divide by the target factor. Mechanical hp derives from 550 ft·lbf/s; metric PS from 75 kgf·m/s. Heat-rate factors divide NIST energy definitions by 3,600 seconds.

For 100 kW, canonical power is 100,000 W. Dividing by 745.69987158227022 gives 134.1022089594 mechanical hp; dividing by 735.49875 gives 135.9621617302 PS after the single output rounding.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NIST SP 811 Appendix B.9 supplies power factors for horsepower, Btu/h, kcal/h and foot-pound-force per second.
2. [2]BIPM SI Brochure defines watt as joule per second and the SI prefix system.
3. [3]NIST SP 330 independently lists watt in the SI derived-unit table.
4. [4]NIST SP 811 Appendix B.8 supplies the energy factors used to independently derive heat-rate units.
5. [5]NIST Handbook 44 Appendix C supplies a customary/SI legal-metrology cross-check.