Efficiency Calculator

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. The tool does not cap the result at 100%: an efficiency above 100% for an ordinary machine means the numbers were swapped or measured differently, since a machine cannot return more energy than it receives. Devices such as heat pumps are rated with a separate coefficient of performance rather than an efficiency.

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.