Energy and power conversion efficiency
Efficiency compares useful output with total input across a clearly defined system boundary. Energy values can be compared with energy values, or steady power values with steady power values, provided both use the same unit and basis.
For passive energy-conversion systems, efficiency cannot exceed 100%. Coefficient of performance for heat pumps and refrigerators is a different metric and can exceed one, so it should not be entered as efficiency.
How to use the efficiency calculator
- Choose the unknown: Select efficiency, useful output, or total input.
- Enter matching quantities: Use energy for both input and output or power for both, in the same unit.
- Calculate: Review the result and confirm efficiency lies between 0% and 100%.
- Define useful output: Document the system boundary, time basis, and which outputs count as useful.
Formula and variables
Rearrange the same relationship to find output or input. Loss equals input minus useful output when both share the same basis.
η = useful output/input × 100%- η — Efficiency
- Useful fraction of input expressed as a percentage (%)
- Eout or Pout — Useful output
- Desired output energy or power
- Ein or Pin — Total input
- Energy or power supplied to the system
Motor output example
A system receives 1,000 W and delivers 800 W of useful output.
- Input
- 1,000 W
- Useful output
- 800 W
- η = 800/1,000 × 100%
- η = 80%
Result: The conversion efficiency is 80%.
Under the stated boundary, 200 W is not counted as useful output and is associated with losses or other outputs.
Understanding your results
Efficiency depends on boundaries
A percentage is meaningful only when input and useful output are measured consistently.
- Changing what counts as useful changes the result.
- Instantaneous power and accumulated energy should not be mixed.
- Measured uncertainty can sometimes produce an apparent value above 100%, signaling a data or boundary problem.
Assumptions
- Input and output use the same physical quantity and unit.
- The system boundary and useful output are defined.
- The values refer to the same operating condition or time interval.
Limitations
- Does not calculate thermal COP, exergy efficiency, uncertainty, parasitic loads, or time-varying performance.
- Does not identify individual loss mechanisms.
- Not a certified equipment-rating calculation.
Common mistakes
- Mixing energy with power.
- Entering efficiency as 0.8 when the field expects 80%.
- Ignoring auxiliary input power.
- Calling COP above one an energy efficiency above 100%.
Practical use cases
Conversion-system screening
Compare useful output with supplied input for a defined operating point.
Physics education
Rearrange the efficiency relationship and account for losses.
Frequently asked questions
Can efficiency exceed 100%?
Not for a passive energy-conversion balance with a complete boundary. A value above 100% indicates mismatched data, missing inputs, or use of a different metric.
Can I use joules and watts together?
No. Convert both quantities to energy over the same interval or use both as power.
Is heat-pump COP an efficiency?
It is a performance ratio with a different definition and may exceed one because it includes heat moved from the environment.
Sources and review
- Energy and Power — OpenStax University Physics Volume 1. Accessed 2026-07-13.
Reviewed 2026-07-13.