Efficiency is the capability of acting or producing effectively with a minimum amount or quantity of waste, expense, or unnecessary effort. The term has widely variant meanings in different disciplines. See
- Algorithmic efficiency for its use in computer science.
- Efficiency (economics) for its use in economics.
- Textual efficiency for its use in linguistics.
- Efficiency (statistics) for its use in statistics.
- Energy efficiency in physics, engineering, and energy management.
- Energy efficiency in relation to economics and the environment, see Energy conservation.
- Electrical efficiency for its use in electronics and electrical engineering.
- Fuel efficiency for its use in automobiles (also known as "gas mileage").
- Lighting efficiency for comparison of different electric light technologies.
- Mechanical efficiency for its use in engineering.
- Material efficiency for its use in engineering, design, and manufacturing
- In rocket propulsion, specific impulse is a measure of mass-efficiency, though a very high specific impulse is not energy-efficient
- Thermodynamic or "engine" efficiency for its use in Thermodynamics, Statistical Physics, related fields, and engines and motors.
- Volumetric efficiency for its use in engine design
- efficiency (fair division), for its use in fair division problems
In several of these cases, efficiency can be expressed as a result as percentage of what ideally could be expected, hence with 100% as ideal case. This does not always apply, not even in all cases where efficiency can be assigned a numerical value, e.g. not for specific impulse.
A slightly broader model of efficiency that nevertheless remains consistent with the "percentage" definition in many cases is to say that efficiency corresponds to the ratio r=P/C of the amount P of some valuable resource produced, per amount C of valuable resources consumed. This may correspond to a percentage if products and consumables are quantified in compatible units, and if consumables are transformed into products via a conservative process. For example, in the analysis of the energy efficiency of heat engines in thermodynamics, the product P may be the amount of useful work output, while the consumable C is the amount of high-temperature heat input. Due to the conservation of energy, P can never be greater than C, and so the efficiency r is never greater than 100% (and in fact must be even less at finite temperatures).