Motor Power Calculator
Calculate motor power from torque and RPM, convert between horsepower and watts, and find electrical input power using motor efficiency.
Calculate Motor Power
Enter torque, RPM, and motor efficiency to calculate mechanical horsepower and electrical power.
How to Calculate Motor Power
Motor power in watts is calculated from torque (Nm) and rotational speed (RPM):
This gives mechanical output power. To find electrical input power, divide by motor efficiency: Pelectrical = Pmechanical ÷ η.
HP to Watts Conversion Table
| Horsepower | Watts | kW |
|---|---|---|
| ¼ HP | 186 W | 0.186 kW |
| ½ HP | 373 W | 0.373 kW |
| ¾ HP | 560 W | 0.560 kW |
| 1 HP | 746 W | 0.746 kW |
| 2 HP | 1,492 W | 1.492 kW |
| 5 HP | 3,730 W | 3.730 kW |
| 10 HP | 7,460 W | 7.460 kW |
| 25 HP | 18,650 W | 18.650 kW |
Motor Efficiency by Size
Larger motors are generally more efficient:
- Fractional HP (< 1 HP): 50-80% efficiency
- 1-5 HP: 78-88% efficiency
- 5-50 HP: 85-93% efficiency
- 50-500 HP: 90-96% efficiency
- Premium efficiency (NEMA): 2-4% higher than standard
Frequently Asked Questions About Motor Power
How many watts is 1 horsepower?
1 mechanical horsepower equals 746 watts (745.7 W precisely). This is the standard conversion used for electric motors. 1 HP = 746 W = 0.746 kW = 33,000 ft·lbs/min.
How do I calculate motor torque from watts?
Torque (Nm) = Power (W) × 60 ÷ (2π × RPM) = 9.5488 × P/RPM. A 1,000W motor at 1,750 RPM produces 5.46 Nm of torque.
What is motor efficiency?
Motor efficiency is the ratio of mechanical output power to electrical input power. A 90% efficient motor drawing 1,000W of electricity produces 900W of mechanical power. The remaining 100W is lost as heat in the windings and bearings.
How do I convert kW to HP?
Divide kilowatts by 0.746: HP = kW ÷ 0.746. A 5.5 kW motor is 5.5 ÷ 0.746 = 7.37 HP.
Why does a motor draw more amps at startup?
At startup, the motor has no back-EMF (counter-electromotive force) to oppose current flow. Full supply voltage drives current through the low winding resistance, resulting in inrush current 5-8× the rated full-load current. As the motor accelerates, back-EMF builds up and current drops to normal.