How do you calculate motor efficiency?

Divide output (shaft) power by input (electrical) power and multiply by 100. With 90 kW out and 95 kW in, efficiency is 90 ÷ 95 × 100 = 94.74%.

What is a good motor efficiency value?

It depends on size and IE class. A mid-size IE3 motor typically runs in the low-to-mid 90s percent at rated load, and IE4/IE5 push higher. The 94.74% in the example is a strong figure for many frame sizes but may fall short of a 95% target.

What does the efficiency gap to target mean?

It is measured efficiency minus target. The example gives 94.74% against a 95% target, a gap of about -0.26 points, meaning the unit is just under target and should be reviewed before it ships as that class.

Why is my motor efficiency below the nameplate rating?

Common causes are higher-than-rated losses: copper losses from winding resistance, core losses, friction and windage, and stray load loss. Test conditions like elevated temperature or off-rated load also pull measured efficiency down.

Does efficiency change with load?

Yes. Most motors peak near 75-100% of rated load and drop off sharply at light loads. A single-point measurement like this one is only valid at the load it was taken; verify at the rated operating point for nameplate compliance.

Input vs output power for efficiency, which goes on top?

Output power (shaft) is always the numerator and input power (electrical) the denominator, because efficiency can never exceed 100%. If your ratio is over 100%, you have swapped the inputs or have a measurement error.

Motors, Generators & Electrification Equipment calculator

Motor efficiency Calculator

Motor efficiency is the ratio of mechanical shaft power out to electrical power in, expressed as a percentage, and it directly governs operating cost and IE-class compliance. Motor test engineers and design teams use it to verify that a build hits its nameplate rating and to quantify how far a unit sits from its target. Because energy cost dominates a motor's lifetime spend, even a fraction of a point matters across a fleet of running hours. This calculator returns both the measured efficiency and the signed gap to your target, so you can see at a glance whether a unit passes or needs investigation.

What this calculator does

Calculate motor efficiency from measured output and input power so energy and test engineers can verify nameplate efficiency and compare it to the target.
Use it when you need a clean motor efficiency percentage and gap to target from a dynamometer or power-analyzer test.
It computes a motor's efficiency from measured output and input power and reports the gap to your target efficiency in percentage points.

Formula used

Motor efficiency = motor output power ÷ motor input power × 100
Efficiency gap to target = motor efficiency - target motor efficiency

Inputs explained

Motor shaft (output) power:
Motor electrical (input) power:
Target nameplate efficiency:

How to use the result

Use it during dynamometer or EOL testing to verify a unit against its nameplate IE class, or when comparing design iterations.
It is a simple output-over-input ratio at one operating point; it does not account for load curve, temperature, or the measurement uncertainty that separates indirect from calibrated loss-segregation methods.

Current U.S. benchmarks

The producer price index for copper and brass mill shapes stands at 559.593 (BLS, May 2026), up 76.8% from a year earlier. Quotes priced off last quarter's material cost miss this move. Global copper trades at $13,484 per tonne (IMF via FRED, May 2026).
Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% 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

How do you calculate motor efficiency? Divide output (shaft) power by input (electrical) power and multiply by 100. With 90 kW out and 95 kW in, efficiency is 90 ÷ 95 × 100 = 94.74%.
What is a good motor efficiency value? It depends on size and IE class. A mid-size IE3 motor typically runs in the low-to-mid 90s percent at rated load, and IE4/IE5 push higher. The 94.74% in the example is a strong figure for many frame sizes but may fall short of a 95% target.
What does the efficiency gap to target mean? It is measured efficiency minus target. The example gives 94.74% against a 95% target, a gap of about -0.26 points, meaning the unit is just under target and should be reviewed before it ships as that class.
Why is my motor efficiency below the nameplate rating? Common causes are higher-than-rated losses: copper losses from winding resistance, core losses, friction and windage, and stray load loss. Test conditions like elevated temperature or off-rated load also pull measured efficiency down.
Does efficiency change with load? Yes. Most motors peak near 75-100% of rated load and drop off sharply at light loads. A single-point measurement like this one is only valid at the load it was taken; verify at the rated operating point for nameplate compliance.

Last reviewed 2026-05-12.