Motor Efficiency

Motor efficiency tells you how much of the electrical energy you put in actually comes out as useful mechanical work. It is the output power divided by the input power, usually shown as a percentage. For example, if a motor takes 1000 watts from the supply and delivers 850 watts of turning power, its efficiency is 85%. The remaining 15% is lost as heat, friction, and noise. Higher efficiency means lower electricity bills and less heat.

Written by the Vorzep Team

Electric Machines Efficiency

Motor Efficiency Calculator

Efficiency, losses, part-load behaviour and annual energy cost for DC, single-phase and three-phase motors.

Loss breakdownEfficiency vs loadPower factorEnergy savings

η = P_out / P_in

Inputs

Solve for

Power unit

Results

Variables

Symbol	Quantity	Unit
η	Motor efficiency	dimensionless (0 to 1)
P_out	Output power (mechanical)	W
P_in	Input power (electrical)	W

What it means

Motor efficiency η is the ratio of mechanical output power to electrical input power, expressed as a percentage. η = (P_out / P_in) × 100%. Losses include copper losses (I²R in windings), core losses (hysteresis and eddy currents), mechanical losses (friction and windage), and stray losses. High‑efficiency motors are essential for energy savings. Motor efficiency varies with load; it is highest near full load. The efficiency is used to size motors and to compare their performance. Example: An electric motor draws 2000W of electrical power and delivers 1700W of mechanical power. Its efficiency is (1700/2000)*100 = 85%. The remaining 15% is dissipated as heat, which must be removed by cooling.

Worked example

Motor Efficiency – Practical Example

Real‑World

Scenario: A motor takes 500 W of electrical power and delivers 400 W of mechanical power. Find efficiency.

Parameter	Value
P_in	500 W
P_out	400 W
Formula	`η = (P_out / P_in) × 100%`

1η = 400 / 500 = 0.8 → 80%

Final Design η = 80% ✓ Good efficiency

Why: Efficiency measures how much input power is converted to useful work – the rest is lost as heat.

Common mistakes

Watch unit consistency and the assumptions behind the formula; misapplying it outside its valid conditions is the most frequent error.

Applications

Motor efficiency η = P_out / P_in is the ratio of mechanical output power to electrical input power. It indicates how much of the input energy is converted to useful work. Engineers use it to select motors, to reduce energy costs, and to comply with efficiency standards. By measuring efficiency, they can identify losses and improve motor design. This formula is essential for energy management and sustainability.

Motor selection for energy‑efficient applications
Energy audits and cost reduction projects
Compliance with motor efficiency standards (IE codes)
Loss analysis and motor design improvement
Educational understanding of motor efficiency

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Published July 10, 2026
Last updated July 14, 2026