Motors are rated in kilowatts in most of the world and in horsepower in North America. Both measure the mechanical power at the shaft, so the conversion is a fixed ratio. What is not fixed is which horsepower you mean, and how much electrical power the motor draws to deliver it.
Formulas
kW = hp × 745.7 / 1000
Input kW = shaft kW / efficiency
One mechanical (imperial) horsepower is 745.7 W, so 1 kW is 1.341 hp. Metric horsepower, written PS or CV, is 735.5 W, so 1 kW is 1.360 PS. The difference is about 1.4 %, which matters when you read a European nameplate.
Worked example
A 7.5 kW motor is 7,500 / 745.7 = 10.06 hp, usually called a 10 hp motor.
A 10 hp motor delivers 10 × 745.7 = 7,457 W at the shaft. If its efficiency is 90 %, the electrical input is 7.457 / 0.9 = 8.29 kW.
Common ratings
| kW | Mechanical hp | Metric PS |
|---|---|---|
| 0.75 | 1.01 | 1.02 |
| 1.5 | 2.01 | 2.04 |
| 2.2 | 2.95 | 2.99 |
| 5.5 | 7.38 | 7.48 |
| 7.5 | 10.06 | 10.20 |
| 11 | 14.75 | 14.96 |
| 22 | 29.50 | 29.91 |
| 37 | 49.62 | 50.31 |
| 75 | 100.58 | 101.97 |
In the other direction, 1 hp is 0.75 kW, 5 hp is 3.73 kW, 10 hp is 7.46 kW, 25 hp is 18.64 kW, 50 hp is 37.28 kW and 100 hp is 74.57 kW.
From horsepower to amps
Horsepower rates the shaft, but the supply carries the electrical input, and a motor also has losses and a power factor. For sizing under the NEC, do not calculate the current from horsepower. Use the full-load current in Table 430.250, which the motor circuit calculator reads for you. A 10 hp three-phase motor on 460 V has a table full-load current of 14 A. For a measured or nameplate load, use the kW to amps calculator with the input power and the power factor.
Common mistakes
- Using shaft power as the electrical input. The motor draws more than it delivers. A 90 % efficient motor draws 11 % more than its shaft output.
- Mixing mechanical and metric horsepower. A “10 hp” label in Europe is often 10 PS, which is 7.35 kW, not 7.46 kW.
- Using horsepower for the circuit. The NEC sizes motor conductors and protection from table current, not from a calculated value.
- Forgetting that kW on an IEC nameplate is the shaft output. The input is higher by the efficiency.
Questions
How many kW is 1 hp?
0.7457 kW for mechanical horsepower, or 0.7355 kW for metric horsepower.
How many hp is 1 kW?
1.341 mechanical hp, or 1.360 metric horsepower.
How many amps is a 10 hp motor?
It depends on voltage and phase. On a three-phase 460 V supply the NEC table value is 14 A, and on 230 V it is 28 A.
Is a 7.5 kW motor a 10 hp motor?
Yes, within about 0.6 %. 7.5 kW is 10.06 hp.
Related calculators
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