By Zencalcy Editorial TeamLast reviewed About us
No real device converts all of its input energy into useful output; the rest is lost, usually as heat. Efficiency compares the useful output with the total input and expresses the result as a percentage. An electric motor might reach 90%, a petrol engine about 25–35%, and an incandescent bulb only around 5% as light.
Useful output as a percentage of energy input.
η = (E_out / E_in) × 100%
η is efficiency as a percentage, E_out is useful output and E_in is total input, both in the same unit. A result above 100% is not possible for a passive device; for a heat pump it reflects a coefficient of performance rather than efficiency.
| Symbol | Meaning |
|---|---|
| η | Efficiency, as a percentage. |
| E_out | Useful output energy or power. |
| E_in | Total input energy or power, in the same unit. |
Efficiency is 800 / 1,000 × 100% = 80%, and 200 J is lost as heat. An 80% efficient device wastes one fifth of every unit of energy it consumes.
Continue with the Elastic Potential Energy Calculator, Kinetic Energy Calculator, or the Mechanical Energy Calculator — or browse more work, energy and power tools and all physics calculators.
Not for a device that only converts energy. Heat pumps can deliver more heat than the electrical energy they consume because they move heat rather than create it; that ratio is a coefficient of performance, not an efficiency.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026