Efficiency
The ratio of useful work output to total energy input for a machine or system.
Governing formula
η = (W_output / E_input) × 100%
SI unit
%
In depth
Due to non-conservative friction and thermal energy dissipation, real machine efficiency is always strictly less than 100%.
Examples in the real world
An electric motor converting 90% of electrical energy input into mechanical work, with 10% lost as heat.