MOTORS · CALCULATOR

Motor efficiency calculator

Free motor efficiency calculator: get efficiency from measured shaft power and measured electrical input. The formula is shown; verify against the NEC.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Measured shaft power

Measured electrical input

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

η = Pout / Pin × 100

Compare measured powers at the same operating point; output cannot exceed input.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Measured shaft power: 4.5 kW · Measured electrical input: 5 kW

Efficiency: 90 %

This example uses the stated inputs; results for the entered values appear above.

Explore the motors branch →See the related electrical concepts →

Estimating and invoicing tool

Ampxa is developing an estimating and invoicing application for electrical contractors. The list below receives notice when it opens.