Efficiency: how much of what you put in comes out useful

Every machine takes energy in and gives energy out. Some of the output does the job you wanted; the rest is dissipated, usually warming the surroundings.

A filament lamp fed with $100\ \text{J}$ might radiate $5\ \text{J}$ as visible light and warm the room with $95\ \text{J}$. Lighting the room is the job, so only $5\ \text{J}$ is useful.

Efficiency is the single number that captures how well the machine does its job.

"Useful" depends on what the device is for:

• an electric motor: the kinetic energy it gives the load is useful; the thermal energy warming its coils is wasted;
• an electric heater: the thermal energy delivered to the room is useful, so a heater can be almost $100\%$ efficient;
• a lamp: the light is useful, even though far more thermal energy comes out.

Always ask: what was the machine built to do?