Efficiency

Efficiency measures how much of the input energy is usefully transferred:

$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}}$

Efficiency is always less than 1 (or 100%) for real machines because some energy is always dissipated as thermal energy.

Both energy and power can be used in the calculation, but the numerator and denominator must use the same quantity. An efficiency above 100% indicates a calculation error (usually an inverted fraction).