Efficiency CalculatorFree online science calculator — instant results, no sign-up
Calculate the energy efficiency of a machine or process.
About the Efficiency Calculator
The Efficiency Calculator measures how much of the energy supplied to a machine or process becomes useful energy output. It uses energy values in joules to calculate both percentage efficiency and energy wasted. With the default useful output of 800 J and input of 1000 J, it gives 80% efficiency and 200 J of wasted energy.
- Useful energy output (J)
- The energy delivered for the machine’s or process’s intended purpose, measured in joules (J), with a default of 800 J.
- Energy input (J)
- The total energy supplied to the machine or process, measured in joules (J), with a default of 1000 J.
Formula and method
η = (E_useful / E_in) × 100%; E_wasted = E_in − E_useful
- ηEnergy efficiency, expressed as a percentage.
- E_usefulUseful energy output in joules (J).
- E_inTotal energy input in joules (J).
- E_wastedEnergy not converted into the specified useful output, in joules (J).
Divide useful energy output by energy input and multiply by 100 to obtain percentage efficiency. Subtract useful energy output from energy input to obtain energy wasted. With the default inputs, η = (800 / 1000) × 100% = 80%, and E_wasted = 1000 − 800 = 200 J.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Useful energy output800 J
- Energy input1000 J
Results
- Efficiency80%
- Energy wasted200 J
How to use the Efficiency Calculator
- 1Define the machine or process boundary and the useful output you want to measure.
- 2Enter the useful energy output in joules; the default is 800 J.
- 3Enter the total energy input in joules for the same operating interval; the default is 1000 J.
- 4Check that the input is greater than zero and that useful output lies between zero and the input.
- 5Run the calculation and read the efficiency and wasted-energy results; the defaults give 80% and 200 J.
Reading your result
Efficiency shows the percentage of supplied energy converted into the chosen useful output: 80% means 80% of the 1000 J input becomes useful output. Energy wasted shows the remaining energy in joules: 200 J in the default example. This energy is not destroyed; it typically leaves in non-useful forms such as dissipated heat or sound.
Limitations
Both energy measurements must use joules and cover the same system boundary and operating interval. Energy input must be positive, and useful output should not exceed input for an ordinary energy-conversion efficiency calculation. Interpreting the difference as wasted energy assumes no net energy storage within the system and no omitted energy inputs or useful outputs. Measurement uncertainty and the definition of 'useful' affect the result. This tool calculates energy efficiency, not power, cost savings, or a heat pump’s coefficient of performance.
Common questions
Why do the default inputs give 80% efficiency and 200 J wasted?+
The useful output is 800 J out of a 1000 J input, so the useful fraction is 0.8, or 80%. Subtracting 800 J from 1000 J leaves 200 J not counted as useful output.
Does wasted energy mean energy has disappeared?+
No. Energy is conserved, but some energy becomes forms that do not serve the intended purpose, such as frictional heating in a machine. Heat can instead count as useful output when heating is the intended purpose.
What if useful energy output is greater than energy input?+
The formula would produce efficiency above 100% and negative wasted energy. For ordinary energy conversion, check for missing inputs, inconsistent measurement intervals, unit errors, or energy released from storage. Heat pumps require a different measure because they transfer environmental heat.
Can I enter watts instead of joules?+
Not directly: watts measure power, while these inputs require energy. Convert each power measurement to energy using energy = power × time, with power in watts and time in seconds. Use the same interval for both measurements; the resulting energies are in joules.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.