Free · Thermodynamics and heat calculators 🌡️

Useful Energy & Input Efficiency Percentage

Calculate useful output energy, input energy or conversion efficiency with consistent energy units and a bounded percentage.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

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An input of 100 J with 80 J of useful output has 80% efficiency.

Efficiency = useful output energy / input energy

Enter these known values and leave the other values blank.

This model measures the useful fraction of input energy for a passive conversion process. Input must be positive; useful output is between zero and input.
It is not a heat-pump coefficient of performance, which uses a different accounting boundary and can exceed one.
BTU options use the International Table definition; therm here means 100,000 IT BTU. Other billing therm definitions can differ.
Zero useful output gives zero efficiency. Zero output and zero efficiency do not uniquely determine the input.

No. This energy-fraction model requires useful output to be no larger than total input; check your units and system boundary.

Bookify enforces a positive input and a bounded useful-energy fraction, and explicitly uses International Table BTU and therm conversion factors.