Wattsto Horsepower Converter
Convert between watts, kilowatts, mechanical horsepower, metric horsepower, electrical horsepower, and boiler horsepower with instant results.
Wattsto Horsepower Converter
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Formula: 1 mechanical HP = 745.7 W | 1 metric HP (PS) = 735.5 W | 1 electrical HP = 746 W
Worked example โ 150 PS = 147.9 mechanical hp = 110.32 kW
Formula
1 mechanical HP = 745.7 W | 1 metric HP (PS) = 735.5 W | 1 electrical HP = 746 W
Horsepower has multiple definitions based on historical context. Mechanical HP was defined by James Watt (550 ft-lbf/s = 745.7 W). Metric HP uses SI-adjacent units (75 kgf*m/s = 735.5 W). Electrical HP is a round 746 W. Boiler HP is 9,809.5 W. This converter handles all variants through a common base unit of watts.
Worked Examples
Example 1: Car Engine Power Comparison
Problem:A European car is rated at 150 PS (metric HP). Convert to mechanical HP and kilowatts.
Solution:To watts: 150 x 735.49875 = 110,324.8 W To kW: 110,324.8 / 1,000 = 110.32 kW To mechanical HP: 110,324.8 / 745.69987 = 147.9 hp
Result:150 PS = 147.9 mechanical hp = 110.32 kW
Example 2: Electric Motor Specification
Problem:Convert a 5 HP electric motor rating to watts and BTU/h output.
Solution:To watts: 5 x 746 = 3,730 W = 3.73 kW To BTU/h: 3,730 / 0.29307107 = 12,728 BTU/h Note: This is shaft output power; electrical input is higher
Result:5 electrical HP = 3,730 W = 12,728 BTU/h
Frequently Asked Questions
How many watts are in one horsepower?
The answer depends on which type of horsepower you mean. Mechanical (Imperial) horsepower equals 745.7 watts, metric horsepower (PS or CV) equals 735.5 watts, and electrical horsepower equals exactly 746 watts. The differences stem from their distinct historical definitions. James Watt defined mechanical horsepower as 550 foot-pounds per second. Metric horsepower was defined as 75 kilogram-force-meters per second.
What is the difference between mechanical and metric horsepower?
Mechanical horsepower (hp) was defined by James Watt as 33,000 foot-pounds per minute (550 ft-lbf/s), which equals 745.7 watts. Metric horsepower (PS in German, CV in French) is defined as 75 kilogram-force-meters per second, equaling 735.5 watts. Metric HP is about 1.4% smaller than mechanical HP. European car specifications typically use metric HP (PS), while American specs use mechanical HP. A 100 PS engine is approximately 98.6 mechanical HP.
What is boiler horsepower?
Boiler horsepower is a much larger unit than mechanical horsepower, equaling 9,809.5 watts or about 13.15 mechanical HP. It was originally defined as the power needed to evaporate 34.5 pounds of water per hour at 212 degrees F from and at 100 degrees C. This equals approximately 33,475 BTU/h. Though largely obsolete for new equipment, boiler horsepower is still occasionally referenced in steam boiler specifications in North America.
Why do electric motors use a different horsepower definition?
Electrical horsepower is defined as exactly 746 watts, a rounded value chosen for convenience in electrical engineering calculations. The slight difference from mechanical HP (745.7 W) is negligible for practical purposes but simplifies motor nameplate ratings and electrical calculations. When you see a motor rated at 1 HP, it means its output shaft delivers 746 watts of mechanical power, though the electrical input will be higher due to motor efficiency losses.
How do I calculate the horsepower of an engine from torque and RPM?
Mechanical horsepower can be calculated from torque and rotational speed using the formula: HP = (Torque in ft-lbf x RPM) / 5,252. This derives from the definition of horsepower as 33,000 ft-lbf per minute combined with the relationship between torque and rotational work. For metric units: kW = (Torque in Nm x RPM) / 9,549. At 5,252 RPM, horsepower and torque in ft-lbf are numerically equal.
How do I convert kilowatts to horsepower?
Multiply kilowatts by 1.34102 to get mechanical horsepower, since 1 kW is 1,000 W divided by 745.7 W per hp. For metric horsepower, often written PS or CV, multiply kilowatts by 1.35962 instead, because a metric horsepower is the slightly smaller 735.5 W. Going the other way, divide horsepower by those same factors, or simply multiply mechanical horsepower by 0.7457 to land directly in kilowatts.
Why are European car outputs quoted in PS or kW rather than hp?
European type approval documentation states engine output in kilowatts because that is the SI unit of power, while manufacturers continue to publish PS figures for marketing because buyers are used to them. PS, the metric horsepower, is defined as 75 kilogram-force metres per second, which is 735.5 W, whereas the mechanical horsepower used in the US and UK is 745.7 W. The metric figure is therefore about 1.4 percent larger for the same engine, which is why the same car looks slightly more powerful when quoted in PS.
What is the difference between brake horsepower and wheel horsepower?
Brake horsepower is measured at the crankshaft using an engine dynamometer that applies a braking load, so it reflects what the engine itself produces. Wheel horsepower is measured at the driven wheels on a chassis dynamometer and is lower, because the gearbox, driveshafts, differential and tyres all absorb some power. Both are ordinary power figures that convert to watts with the same factor, so a difference between two quoted numbers is a measurement location question, not a unit conversion question.
How do I turn a horsepower rating into the energy a machine consumes over time?
Power multiplied by time gives energy, so convert the horsepower to watts first and then multiply by the running hours. One mechanical horsepower is 745.7 W, so running at 1 hp for one hour delivers 745.7 watt hours, or about 0.7457 kWh. A 5 hp motor running for 8 hours therefore produces about 29.8 kWh of mechanical output. Actual electrical draw is higher, because a motor efficiency below one hundred percent means input power exceeds shaft output.
What is the difference between a continuous and a peak power rating?
A continuous or nameplate rating is the power a machine can sustain indefinitely without overheating, and it is the figure to use when sizing supply cables, generators and cooling. A peak or intermittent rating is what the machine can deliver for a short burst before thermal limits bite, and on electric motors it can be several times higher. Converting watts to horsepower does not change which rating you are looking at, so confirm the duty cycle before comparing two machines by horsepower alone.
References
Background & Theory
1 mechanical horsepower = 745.699872 watts; 1 metric horsepower (PS) = 735.49875 watts exactly; 1 electrical horsepower = 746 watts exactly. The three definitions differ by up to 1.4 percent, so an engine rated 150 in one is not rated 150 in another.
Conversion reference table
| Conversion | Equivalent | Exact or rounded? |
|---|---|---|
| 1 mechanical/imperial hp | 745.699872 watts (550 ft-lbf/s) | Derived, rounded |
| 1 metric horsepower (PS, CV) | 735.49875 watts (75 kgf-m/s) | Exact by definition |
| 1 electrical horsepower | 746 watts | Exact by convention |
| 1 boiler horsepower | 9,809.5 watts | Defined, rounded |
| 1 kilowatt | 1.34102209 mechanical hp | Derived, rounded |
| 1 kilowatt | 1.35962162 metric hp (PS) | Derived, rounded |
European manufacturers quote PS or CV, the metric horsepower defined as the power needed to raise 75 kilograms by one metre in one second; British and American figures use the mechanical horsepower James Watt defined as 33,000 foot-pounds per minute. Because PS is the smaller unit, the same engine always carries a slightly larger PS number - 150 PS is 147.9 mechanical hp, or 110.3 kW. The kilowatt is the unambiguous figure and is what type-approval documents record.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator ยท Editorial policy
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