Efficiency Calculator
Calculate mechanical or thermal efficiency as a percentage from useful output and total input energy.
How it works
- 1Enter the useful energy output — the portion of energy that does the intended work.
- 2Enter the total energy input supplied to the system (in the same unit, e.g. joules or watts).
- 3The calculator applies η = useful output ÷ total input × 100% and shows efficiency, wasted energy, and wasted percentage.
Use cases
- Comparing electric-motor efficiency ratings to choose the most energy-efficient model.
- Evaluating heat-engine performance against the theoretical Carnot limit.
- Auditing industrial machinery to quantify energy losses and target improvements.
Frequently asked questions
What is the efficiency formula?
Efficiency η = (useful output ÷ total input) × 100%. For example, a motor that receives 100 J and delivers 80 J of useful work is 80 ÷ 100 × 100% = 80% efficient. The other 20 J is wasted, usually as heat.
Why can efficiency never exceed 100% for a real machine?
Every real conversion loses some energy as heat, sound, or friction — required by the second law of thermodynamics. The useful output can never equal or exceed the total input; a result above 100% would mean energy created from nothing.
Does it matter which unit I use?
No — efficiency is a dimensionless ratio. As long as you use the same unit for both useful output and total input (joules, kWh, BTU, watts, …), the percentage is identical.
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