5.1.3—That the efficiency of a system is the ratio of useful energy output

Efficiency is η=useful output energy ÷ total input energy, often expressed as a percentage.

Use the same quantity type on top and bottom: useful energy with input energy, or useful power with input power. The remainder is not destroyed.

A motor receiving 500 J and delivering 350 J of useful mechanical energy has efficiency 0.70, or 70%.

Efficiency cannot exceed 100% for a passive system, and useful output is not necessarily the whole output.