Why can efficiency never exceed 100%?

A motor cannot deliver more mechanical power than the electrical power it draws, because the difference is loss in copper, iron and windings. A reading above 100% means the two power measurements are on different bases or one is wrong.

How does a horsepower nameplate fit this page?

Convert it first: one horsepower is 0.746 kilowatts, so a 100 hp motor is 74.6 kW of rated output. Enter the converted figure, never the horsepower number in a kilowatt field.

Is percent of rated output the same as load factor?

No. Percent of rated output is this reading against the nameplate rating. Load factor averages load over a period against peak load, a different comparison that uses logged demand rather than one operating point.

Why warn at the nameplate target?

The target is the efficiency class the motor is supposed to meet. When the measured point sits below it, either the load is off the sweet spot or the motor needs service, and both are worth knowing.

Motors, Generators & Electrification Equipment calculator

Motor Efficiency Calculator

Measure how efficiently a motor converts electrical power into shaft work at the operating point you enter. Real kilowatts go in, shaft kilowatts come out, and the difference is loss.

What this calculator does

Motor efficiency and its gap to the nameplate target, with real power loss and the share of rated output.

Formula used

Motor efficiency = shaft output ÷ electrical input × 100
Efficiency gap to target = target − efficiency (positive means below target)
Power loss = electrical input − shaft output
Percent of rated output = shaft output ÷ rated output × 100

Inputs explained

Shaft Output Power: Mechanical power the shaft delivers, in real kilowatts.
Electrical Input Power: Real power the motor draws at the terminals, in kilowatts.
Target Nameplate Efficiency: Nameplate efficiency class the motor should meet, as a percentage.
Rated Output Power: Nameplate rated output, used to place the operating point.

How to use the result

Best suited to checking a motor against its nameplate class, screening a rewind against a new premium motor, locating where motor losses sit at part load.
One operating point cannot describe a motor that runs across a wide load range. Efficiency varies with load, so a part-load reading can sit below the nameplate class.

Current U.S. benchmarks

The producer price index for primary nonferrous metals (a broad metals benchmark, not copper alone) stands at 544.731 (BLS, Aug 2026), up 49.3% from a year earlier. Quotes priced off last quarter's material cost miss this move. Global copper trades at $13,543 per tonne (IMF via FRED, Jul 2026).
Steel mill PPI stands at 381.162 (BLS, Aug 2026), up 23.4% from a year earlier. New factory orders are up 8.5% year over year (Census).
The U.S. has 5,397 electrical equipment and appliances establishments employing about 369,437 workers (Census County Business Patterns, 2023).

Common questions

Why can efficiency never exceed 100%? A motor cannot deliver more mechanical power than the electrical power it draws, because the difference is loss in copper, iron and windings. A reading above 100% means the two power measurements are on different bases or one is wrong.
How does a horsepower nameplate fit this page? Convert it first: one horsepower is 0.746 kilowatts, so a 100 hp motor is 74.6 kW of rated output. Enter the converted figure, never the horsepower number in a kilowatt field.
Is percent of rated output the same as load factor? No. Percent of rated output is this reading against the nameplate rating. Load factor averages load over a period against peak load, a different comparison that uses logged demand rather than one operating point.
Why warn at the nameplate target? The target is the efficiency class the motor is supposed to meet. When the measured point sits below it, either the load is off the sweet spot or the motor needs service, and both are worth knowing.

Last reviewed 2026-10-01.