Electric Motor Efficiency Calculator
Electric motor efficiency is the ratio of mechanical shaft output power to electrical input power. A motor rated at 10 hp (7,460 W) with 92% efficiency draws 7,460 / 0.92 = 8,109 W from the electrical supply. The 649 W difference is lost as heat. This calculator computes input power, shaft output power, losses, annual electricity consumption, and annual cost for any motor. It also computes the savings from upgrading to a higher efficiency motor.
Electric motor efficiency formulas (IEEE 112)
Shaft Output (W) = hp x 746 x (Load Factor / 100)
Input Power (W) = Shaft Output (W) / (Efficiency / 100)
Motor Losses (W) = Input Power - Shaft Output
Annual kWh = Input Power (kW) x Annual Hours
Annual Cost = Annual kWh x Rate
Annual Saving = (Old Input kW - New Input kW) x Hours x Rate
IEEE Standard 112-2017 (Test Procedure for Polyphase Induction Motors and Generators) defines the standard efficiency test method. NEMA MG-1 (Motors and Generators) specifies minimum efficiency levels by frame size and horsepower rating. DOE 10 CFR Part 431 mandates NEMA Premium efficiency for most new motors above 1 hp.
NEMA motor efficiency grades
- NEMA Premium (MG-1 Table 12-12): highest efficiency tier, required by DOE for most new motors since 2010.
- NEMA Energy Efficient (MG-1 Table 12-11): previous minimum federal standard, now superseded for most sizes.
- Standard efficiency: older motors pre-2010, typically 85-90% for mid-size motors.
- Rewound motors typically lose 1-2 percentage points of efficiency per rewind; replacement with NEMA Premium is usually more economical for motors larger than 50 hp.
Electric motor efficiency: frequently asked questions
How is electric motor efficiency calculated?
Motor efficiency = (Mechanical shaft output power / Electrical input power) x 100%. Equivalently, Output Power (W) = Input Power (W) x Efficiency (%). The difference between input and output is heat loss in the motor windings, iron core, friction, and windage. This is the standard definition per IEEE Standard 112 (Test Procedure for Polyphase Induction Motors).
What are NEMA Premium efficiency motors?
NEMA (National Electrical Manufacturers Association) defines efficiency grades for induction motors. NEMA Premium motors must meet minimum efficiency levels specified in NEMA MG-1 Table 12-12 (typically 91-96% depending on horsepower). The DOE mandates NEMA Premium efficiency for most general-purpose motors sold in the US under 10 CFR Part 431.
How do I convert horsepower to kilowatts?
1 mechanical horsepower (hp) = 746 watts = 0.746 kW. A 10 hp motor has a nameplate output of 7,460 W. To find electrical input power, divide shaft output by motor efficiency: Input (W) = Output (W) / Efficiency.
What is motor load factor and why does it matter?
Load factor is the ratio of actual mechanical load on the motor to its rated capacity. A motor running at 50% load factor draws about 60-70% of full-load current and typically operates at reduced efficiency. Most motors are most efficient between 75-100% of rated load. The DOE Motor Systems Guide recommends sizing motors to run at 75-100% load for best efficiency.
How much can upgrading to a high-efficiency motor save?
The DOE Industrial Technologies Program estimates that upgrading from a standard efficiency motor (88-90%) to a NEMA Premium motor (94-96%) saves 4-8% on electricity costs for that motor. For a continuously running industrial motor, this can represent thousands of dollars per year. Payback periods for motor upgrades are typically 1-3 years.
Official sources
- DOE Office of Industrial Technologies: Motor Systems (DOE Advanced Manufacturing Office).
- DOE 10 CFR Part 431: Energy Efficiency Standards for Electric Motors.
- NEMA MG-1: Motors and Generators standard (nema.org). Referenced in DOE regulatory text.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.