Efficiency
Definition
How much of the energy you put into a machine comes out as useful work — the rest is wasted, mostly as heat. No machine is 100% efficient.
Symbol: · Unit: dimensionless — often written as a percent
Technical definition
The ratio of useful energy (or power) output to total energy (or power) input: η = E_useful/E_input, always less than 1. The second law of thermodynamics caps heat-engine efficiency at the Carnot limit, 1 − T_cold/T_hot.
Example
A typical gasoline car engine converts only about 25–30% of fuel energy into motion; the rest leaves as exhaust heat. An LED bulb turns roughly 40–50% of electrical energy into light, versus about 5% for an old incandescent.
Efficiency is not the same as power. A 2000-watt heater and a 1000-watt heater can both be 100% efficient at turning electricity into heat — the bigger one is more powerful, not more efficient. Efficiency compares useful output to input, never raw size.