What this tool does
This tool converts power measurements from watts to four different definitions of horsepower: mechanical (Imperial), metric (PS/CV), electrical, and boiler horsepower. Since horsepower has multiple definitions that differ by up to 1.4%, using the wrong conversion factor can lead to specification errors. This tool shows all four conversions simultaneously so users can select the appropriate value for their application. The watt is the SI unit of power, while horsepower remains widely used in automotive, industrial, and HVAC applications.
How it calculates
Each horsepower variant uses a different conversion factor from watts. Mechanical horsepower: hp = W / 745.7 (defined as 550 foot-pounds per second). Metric horsepower: PS = W / 735.499 (defined as 75 kilogram-force meters per second). Electrical horsepower: hp = W / 746 (used for electric motor ratings). Boiler horsepower: hp = W / 9,809.5 (used for steam boiler output). The tool divides the input wattage by each factor to produce all four results simultaneously.
Who should use this
- **Automotive engineers** converting electric motor output from watts to horsepower for specifications - **HVAC technicians** sizing equipment rated in both watts and horsepower - **Industrial engineers** comparing motor specifications across different rating systems - **EV enthusiasts** understanding electric motor power in familiar horsepower terms
Worked examples
Example 1: A Tesla Model 3 motor produces 211,000 watts. In mechanical HP: 211,000 / 745.7 = 282.9 hp. In metric HP: 211,000 / 735.499 = 286.8 PS. Example 2: A 1,500W space heater equals 1,500 / 745.7 = 2.01 mechanical HP. Example 3: A 5 HP pump motor consumes 5 × 746 = 3,730 watts (using electrical HP definition).
Limitations
This tool converts between power units only and does not account for motor efficiency, which causes the input power (watts consumed) to differ from output power (watts or HP delivered). The mechanical and metric horsepower definitions differ by approximately 1.4%, which can be significant in precision applications. The tool uses standard conversion factors and does not account for regional variations in horsepower definitions.
FAQs
Q: Which horsepower should I use? A: In the US and UK, mechanical horsepower (745.7 W) is standard. In Europe and Asia, metric horsepower (PS/CV/pk at 735.499 W) is more common. For electric motors, electrical horsepower (746 W) is the standard. Boiler horsepower is used only for steam boilers.
Q: Why are there different definitions of horsepower? A: Historical reasons. James Watt defined mechanical HP based on the work output of draft horses. Different countries later adopted slightly different definitions. The metric horsepower was defined using metric units (kgf and meters) resulting in a slightly different value.
Q: Is 1 HP exactly 746 watts? A: Only electrical horsepower is exactly 746 watts. Mechanical horsepower is 745.69987... watts. Metric horsepower is 735.49875... watts. The differences are small but can matter in precision engineering.
Q: How do I convert kilowatts to horsepower? A: Multiply kilowatts by 1,000 to get watts, then divide by 745.7 for mechanical HP. Alternatively, divide kilowatts directly by 0.7457. For example, 100 kW = 134.1 mechanical HP.
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