Power Converter
Power is the rate at which energy is used or produced, and it is measured in many different units across various fields and countries. The SI unit is the watt (W), equal to one joule per second, familiar to anyone paying an electricity bill. A typical light bulb uses 60 watts, a hair dryer uses 1,500 watts, and a house's peak load might be 10 kilowatts. Horsepower (hp), an imperial unit equal to about 746 watts, originated from comparing engine power to horses and is still used for vehicle engines and mechanical equipment in the United States. The metric horsepower (PS, from the German 'Pferdestärke'), equal to about 735 watts, is used in some countries for vehicle ratings. In HVAC systems, power is expressed as BTU per hour, with one BTU per hour equalling about 0.293 watts. Older units like calories per second and foot-pounds per second persist in specialized contexts. Converting between these units is essential when comparing equipment power ratings, understanding energy bills, evaluating engine performance specifications, or estimating heating and cooling capacity. A 100-watt light bulb, a 0.134-horsepower motor, and a 341-BTU/hour heat output are all equivalent. This calculator displays ten power units: milliwatts through foot-pounds per second. Enter a value in any field and all others update instantly with NIST precision.
NIST conversion factors
All conversions below use standard NIST-defined factors, with all units expressed relative to the watt.
| Unit | Symbol | Watts |
|---|---|---|
| Milliwatt | mW | 0.001 |
| Watt | W | 1 |
| Kilowatt | kW | 1,000 |
| Megawatt | MW | 1,000,000 |
| Gigawatt | GW | 1,000,000,000 |
| Horsepower | hp | 745.69987 |
| Metric horsepower | PS | 735.499 |
| BTU per hour | BTU/h | 0.29307 |
| Calorie per second | cal/s | 4.184 |
| Foot-pound per second | ft.lbf/s | 1.355818 |
Power converter: frequently asked questions
What is the SI unit of power?
The watt (W) is the SI unit of power, defined as one joule per second. One watt is the power of a one-amp current at one volt. A typical light bulb uses 60 watts, while a hair dryer uses about 1,500 watts. Electricity consumption is measured in kilowatts (kW) or megawatts (MW).
What is horsepower?
Horsepower (hp) is an imperial unit equal to about 746 watts. It originated from comparing the power output of engines to horses. One horsepower can lift 550 pounds one foot in one second. A metric horsepower (PS) equals about 735 watts, and is used in some countries.
What is a BTU per hour?
BTU per hour is the power unit used in HVAC systems in the United States. One BTU per hour equals about 0.293 watts. Air conditioning capacity is rated in BTUs per hour, while heating furnace output is measured in BTU/h.
How precise are these conversions?
These use NIST-defined conversions. One watt equals exactly one joule per second. One horsepower equals exactly 745.699872 watts. All conversions shown are precise to two decimal places.
Why would I need to convert power units?
Different fields use different power units. Electricity uses watts/kilowatts. Engines use horsepower. HVAC uses BTU/hour. Different countries use different mechanical power units. Converting ensures clarity when comparing equipment specifications, understanding energy consumption, or evaluating system performance.
Official sources
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices.
- NIST Special Publication 330: The International System of Units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.