Formula
Energy efficiency = useful energy output ÷ total energy input × 100Direct answer
Divide useful energy output by total energy input and multiply by 100. The U.S. Department of Energy describes energy efficiency as output over input in most cases. This calculator makes that ratio explicit for one stated system boundary.
Variables and formula
- Useful energy output: the energy that performs the intended service.
- Total energy input: energy supplied to the same system boundary, in the same unit.
Energy efficiency = useful energy output ÷ total energy input × 100
Reproducible example
If a process has 800 compatible energy units of useful output from 1,000 input units, efficiency is 800 ÷ 1,000 × 100 = 80%. The uncounted 200 energy units are shown as losses in this simple model.
Interpretation
An 80% result says that, within the supplied inputs and boundary, 80% of input energy appears as useful output. It does not label a product efficient or compare products with different duties, climates, test conditions or rating methods.
Assumptions and limitations
- Input and output use compatible units and refer to the same system boundary.
- Useful output cannot exceed input in this simple loss-accounting model.
- Product ratings, seasonal performance, lifecycle energy and utility charges are outside scope.
Common mistakes
- Mixing kWh with another energy unit without converting one of them.
- Treating nameplate power as useful energy output.
- Comparing different test conditions as though the percentages were directly interchangeable.
Assumptions
- Input and useful output describe the same system boundary and use compatible energy units.
- Useful output does not exceed total input in this simple loss-accounting model.
- Product-specific ratings, seasonal conditions, utility billing and lifecycle energy are excluded.
Sources and methodology
CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.
- Take Five: What Is Energy Efficiency?; U.S. Department of Energy; accessed Aug 27, 2026