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Efficiency Calculator

Find the efficiency percentage.

How it is calculated

Efficiency is η = (useful output / total input) × 100, and the loss is 100 − η. Output and input must be in the same unit (both joules, both watts, both kWh and so on); the tool does no unit conversion. The input must be greater than zero, otherwise nothing is returned. Both figures are shown with two decimals and a percent sign. An output larger than the input (or a negative output) gives no result: a machine cannot return more energy than it receives, so such figures were swapped or measured differently. Devices such as heat pumps are rated with a separate coefficient of performance rather than an efficiency.

Worked example

A motor draws 1000 J of electrical energy and delivers 850 J of mechanical work. Efficiency = 850 / 1000 × 100 = 85.00%. The remaining 100 − 85 = 15.00% is the loss, which leaves the motor mostly as heat.

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Common mistakes

  • Swapping output and input, which produces a value above 100%.
  • Mixing units, for example output in kWh and input in joules, or one value in watts and the other in kilowatts.
  • Entering the lost (wasted) energy as the output instead of the useful energy.
  • Comparing power in one figure with energy in the other; both must be energy or both must be power over the same period.
  • Reading the percentage of loss as if it disappears: it is converted, usually into heat.

About this calculator

This calculator finds efficiency as a percentage: useful output divided by total input, multiplied by 100. It's the same ratio engineers use to rate motors, engines, and machines, and the same logic anyone can apply to any process where some input is "lost" along the way.

Mechanical and electrical engineers use it to rate motors, pumps, and power supplies; students use it for physics and thermodynamics homework; and operations staff use it to measure how much of a process's input actually turns into useful work. Output and input must be in matching units for the percentage to mean anything.

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Comments (1)

  • Editorial team

    Output and input must be in the same unit, such as joules in and joules out, otherwise the percentage means nothing. An output larger than the input gets no result, because no machine returns more energy than it takes in; it usually means the two figures were swapped or measured in different units. Heat pumps look like an exception, but their ratio is a coefficient of performance, not efficiency. Which device are you measuring?

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