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Motor Efficiency

η=100 PoutPin\eta = \frac{100 \, P_{out}}{P_{in}}
sig figs

Worked example: 18.5 kW out of 20 kW in → 92.5%. Press Try an example to run it live, then adjust anything.

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Where Motor Efficiency fits

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Your answer feeds
Three-Phase Motor Full-Load Current
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See also

Three-Phase Motor Full-Load Current: Both use motor efficiency and output power.Motor Torque from Power and Speed: Both use output power.
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Motor Efficiency explained

MηPinPout

The difference between input and output is heat: stator and rotor I²R losses, core losses, windage and friction. A 10 hp motor at 88% efficiency swallows 7457/0.88 ≈ 8474 W to deliver 7457 W of shaft power, dumping about a kilowatt into the room, which is why motor rooms need ventilation and why efficiency shows up twice in an energy audit, once as electricity and once as cooling load.

Efficiency is not constant: it peaks near 75–100% of rated load and falls off a cliff below about 40%, which is the strongest argument against habitually oversizing motors. Since the 1990s, minimum efficiencies have been legislated (IE3 "premium" class in the IEC world, EPAct and NEMA Premium in the US) and the gain from an IE1 to an IE3 machine, a few points, usually repays the price difference within a year of continuous running.

Motor Efficiency formula

η=100 PoutPin\eta = \frac{100 \, P_{out}}{P_{in}}
Where
  • η\eta= Efficiency (%)
  • PoutP_{out}= Output power (W)
  • PinP_{in}= Input power (W)
In this toolkit

Motor calculations

5 formulasFull-load current, synchronous speed, slip, efficiency and input power — reading a motor nameplate and checking what it actually draws.

Revised 30 October 2026
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formula.expert. (2026, October 30). Motor Efficiency Formula & Calculator. https://formula.expert/solvers/motor-efficiency-percent

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