Glossary term
Efficiency
The ratio of useful output to the total input required to produce it, expressed as a dimensionless number or percentage.
Definition
metricEfficiency is the ratio of useful output to the total input consumed, quantifying how effectively a system, process, or device converts or uses resources.
Efficiency is one of the most widely used performance metrics in engineering. It applies to any system that receives an input — energy, work, heat, material, or signal — and produces a desired output. Because real systems always incur losses, efficiency is strictly less than 100% in practice. The appropriate form of efficiency depends on the domain: thermal efficiency for heat engines, isentropic efficiency for turbomachinery components, volumetric efficiency for pumps and compressors, transmission efficiency for mechanical drives. Each of these is treated in a dedicated entry.
Efficiency quantifies how well a system converts its inputs into a desired output. In its most general form:
where \eta (eta) is dimensionless and lies in the interval [0, 1], or equivalently [0\%, 100\%]. A value of \eta = 1 would mean perfect conversion with no losses — a limit that real systems cannot reach. The gap between actual efficiency and unity represents the losses inherent to any physical process: friction, electrical resistance, heat dissipation, leakage, incomplete chemical reaction, or irreversible thermodynamic processes.
Efficiency stacking
In any engineered system, the overall efficiency is the product of the efficiencies of all subsystems in the energy conversion chain. For n stages in series:
Each factor is less than one, so the end-to-end efficiency is always lower than any individual component’s efficiency. This multiplicative relationship shows why long conversion chains accumulate losses rapidly: three stages each at 90% efficiency yield an overall efficiency of only 73%. Identifying the dominant loss mechanism is the first step in any efficiency improvement programme.
Fundamental limits
The second law of thermodynamics places absolute limits on the efficiency of processes involving heat. No heat engine can exceed the Carnot efficiency. No refrigeration cycle can exceed the Carnot coefficient of performance. These are not engineering limitations to be overcome by better design — they are consequences of the irreversibility of entropy production in real processes. Within the thermodynamic limit, engineering constraints — cost, weight, reliability, manufacturing tolerance — set practical bounds that are often much tighter.
Domain-specific forms
The general definition \eta = W_\text{useful}/W_\text{input} takes specific forms depending on the system and the nature of its inputs and outputs. Thermal efficiency applies to heat engines converting heat to work. Isentropic efficiency applies to individual turbomachinery components — turbines, compressors, pumps — comparing actual performance to the ideal reversible adiabatic process. Volumetric efficiency applies to reciprocating machines and quantifies how completely the working volume is filled. Each of these domain-specific metrics is treated in a dedicated entry.