Convert horsepower and watts both ways, choose the horsepower kind, and see the same power in kilowatts.
Table of contents
How to use our Horsepower to Watts Converter
- Use Convert from to choose whether your starting value is horsepower or watts.
- Pick the correct Horsepower kind; mechanical hp is common for US engines, while metric hp, electrical hp, and boiler hp use different factors.
- Enter the Power amount in the chosen starting unit; commas are OK, such as 1,500.
- Open Advanced options if you want to change Round answer to or enter Motor efficiency for a motor input-power estimate.
- Check that Conversion factor used matches your horsepower label, then compare Converted power and Same power in kilowatts with the rating you expected.
Definitions
Power: How fast work is done or energy is used.
Watt: The standard SI unit of power. A 1,000 watt amount is 1 kilowatt.
Horsepower: A power unit often used for engines, motors, and boilers. Its watt value depends on the horsepower kind.
Horsepower kind: The exact horsepower definition selected in the calculator: mechanical, metric, electrical, or boiler.
Kilowatt: A power unit equal to 1,000 watts.
Conversion factor used: The number of watts that equal 1 selected horsepower.
Motor efficiency: The percent of electrical input power that becomes useful shaft output power in a motor [2].
Estimated electrical input power: The estimated wall-side power needed by a motor when shaft output watts and efficiency are known.
Common mistakes and quick fixes
Mistake: Leaving Convert from on Horsepower to watts when the number you have is already in watts.
Fix: Change Convert from to Watts to horsepower before entering the Power amount .
Mistake: Using the wrong Horsepower kind , such as metric hp for a US engine rating.
Fix: Match Horsepower kind to the rating label; use mechanical hp for most US engine horsepower.
Mistake: Typing units or words in Power amount , such as 5 hp or abc.
Fix: Enter only the number in Power amount ; the unit comes from Convert from .
Mistake: Treating Round answer to as if it changes the real conversion.
Fix: Use Round answer to only for display precision; the calculator keeps full precision internally.
Mistake: Entering Motor efficiency as 0, over 100, or with extra text.
Fix: Enter a percent greater than 0 and at most 100, such as 85, or leave Motor efficiency blank.
Mistake: Expecting Estimated electrical input power in Watts to horsepower mode.
Fix: Use Horsepower to watts mode with Motor efficiency when you want a wall-side input watt estimate.
Limitations & Key Assumptions / Boundary Conditions
- The converter rejects blank, non-number, and negative Power amount values because this page is for basic positive power ratings.
- Motor efficiency is optional. If it is blank, the result is a plain unit conversion with no motor-loss estimate.
- The Estimated electrical input power is shown only for Horsepower to watts mode because a watts entry may already be input power.
- The motor input estimate uses one efficiency percent. Real motors can draw different power at startup, at partial load, or when voltage and power factor matter.
- The horsepower factors are fixed: mechanical 745.699872 W per hp, metric 735.49875 W per hp, electrical 746 W per hp, and boiler 9809.5 W per hp.
- Round answer to changes only the displayed number of decimal places. It does not change the stored math used for the conversion.
Methodology
Conversion steps
The calculator first picks the watts-per-horsepower factor from Horsepower kind. Mechanical hp uses 745.699872 W per hp, metric hp uses 735.49875 W per hp, electrical hp uses 746 W per hp, and boiler hp uses 9809.5 W per hp.
watts = horsepower * watts_per_hp
horsepower = watts / watts_per_hp
For Same power in kilowatts, the calculator uses the power in watts. In Horsepower to watts mode, that is the converted watts. In Watts to horsepower mode, that is the entered watts.
kilowatts = watts / 1000
Motor efficiency estimate
If Motor efficiency is entered in Horsepower to watts mode, the calculator treats the converted watts as shaft output power. Motor efficiency is the ratio of shaft output power to electrical input power [1].
input_watts = output_watts / (efficiency_percent / 100)
The efficiency value must be greater than 0 and at most 100. A value below 100 percent makes estimated input watts larger than output watts.
Mini-example
For 5 mechanical horsepower, the output watts are 5 * 745.699872 = 3728.49936 W. The same power is 3728.49936 / 1000 = 3.72849936 kW. With 85 percent motor efficiency, estimated input watts are 3728.49936 / 0.85 = 4386.469835 W.
Rounding
The selected Round answer to value is applied only after the calculation. Large numbers are shown with comma separators, and the calculator avoids scientific notation in visible results.