efficiency

Feed a machine some energy and only part of it comes out doing the job you wanted — the rest slips away, almost always as waste heat. An old light bulb turns most of its electricity into heat and only a little into light; a car engine turns most of its fuel into heat and only some into motion. Efficiency is the simple score for this: what fraction of the energy you put in actually does useful work.

It is defined as efficiency = useful energy output divided by total energy input — or equally as useful power out over total power in — usually quoted as a percentage. A device that draws 100 J and delivers 25 J of useful output is 25 percent efficient. Because energy is conserved, efficiency can never exceed 100 percent: you cannot get more useful energy out than you put in, and in practice you always get less, because some is inevitably dissipated.

You meet efficiency ratings on engines, motors, light bulbs, and power plants. One honest and surprising limit: a heat engine, which turns heat into work, has a theoretical ceiling well below 100 percent set by the Carnot cycle, no matter how cleverly it is built — this is a law of nature, not a lack of engineering. And the wasted energy is never destroyed; it has simply been dissipated into the surroundings as heat, where it is too spread out to be useful.