Motor Power Converter

Motor power conversion factors.

Base conversions:

• 1 HP (mechanical) = 745.7 watts = 0.7457 kW
• 1 kW = 1.341 HP = 1,000 watts
• 1 HP = 2,544.43 BTU/hr
• 1 kW = 3,412.14 BTU/hr

Common motor sizes:

• Small fan: 1/4 HP (186 W)
• Garage door opener: 1/2 HP (373 W)
• Power tools: 1-3 HP (746-2,237 W)
• Car engine: 150-300 HP (112-224 kW)
• Electric car: 200-400 HP (149-298 kW)
• Diesel truck: 300-600 HP (224-447 kW)

Types of horsepower:

• Mechanical/Imperial HP: 745.7 W (used for engines)
• Metric HP (PS): 735.5 W (used in Europe)
• Electrical HP: 746 W (used for electric motors)

Those three flavors of horsepower exist because the unit was adopted by different industries, and the gaps are small but real: a “150 PS” European car works out to about 148 mechanical hp, which is why the same engine can wear two different numbers in two markets.

A more important distinction for electric motors is input versus output power. The watts a motor pulls from the supply aren’t all delivered to the shaft, because no motor is perfectly efficient, so a unit rated “1 kW output” draws somewhat more than 1 kW from the wall, the difference lost as heat. It’s also worth checking whether a rating is continuous or peak. Many motors can briefly produce far more than they can sustain, and mistaking the short-term peak for the continuous rating is a common way to burn out an undersized motor.

The nameplate figure is the SHAFT output, not the draw. This is the single most common mistake when sizing a supply. A 1 HP motor at 80% efficiency delivers 746 W to the shaft while pulling 932 W from the wire, and the 186 W difference comes out as heat inside the housing. Size the breaker, the cable and the ventilation on the input; size the machine on the output.

Typical efficiencies, which vary far more than most people expect:

Small motors are dramatically worse than large ones, and it is not a manufacturing failing but geometry: losses scale with surface area while output scales with volume, so efficiency climbs with size. The practical upshot is that a 1/4 HP motor might draw three times its rated output, while a 100 HP motor draws barely five percent over.

Amps follow from watts, and the formula depends on the phase count. For a single-phase supply the input current is watts ÷ (volts × power factor); for three-phase it is watts ÷ (volts × power factor × 1.732), where 1.732 is the square root of three. Induction motors typically run a power factor between 0.8 and 0.9 at full load and much worse when lightly loaded, which is why an oversized motor costs money twice over: once in purchase and once in the reactive current it pulls for no useful work.

Where the horsepower came from, since somebody always asks. James Watt measured mill horses turning a wheel and settled on 33,000 foot-pounds per minute as what one could sustain. It was a marketing figure, chosen so a customer could count how many horses his steam engine would replace, and he pitched it deliberately high. A real horse peaks well above one horsepower for short bursts and sustains rather less all day, so the unit is named after an animal that does not quite meet it.