Energy conversion efficiency

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Output energy is always lower than input energy
Efficiency of Power Plant, World total 2008

Energy conversion efficiency (η) is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be electric power, mechanical work, light (radiation), or heat.


Energy conversion efficiency is not defined uniquely, but instead depends on the usefulness of the output. All or part of the heat produced from burning a fuel may become rejected waste heat if, for example, work is the desired output from a thermodynamic cycle. Energy converter is an example of an energy transformation. For example a light bulb falls into the categories energy converter. 
\eta = \frac{P_\mathrm{out}}{P_\mathrm{in}}
Even though the definition includes the notion of usefulness, efficiency is considered a technical or physical term. Goal or mission oriented terms include effectiveness and efficacy.

Generally, energy conversion efficiency is a dimensionless number between 0 and 1.0, or 0% to 100%. Efficiencies may not exceed 100%, e.g., for a perpetual motion machine. However, other effectiveness measures that can exceed 1.0 are used for heat pumps and other devices that move heat rather than convert it.

When talking about the efficiency of heat engines and power stations the convention should be stated, i.e., HHV (aka Gross Heating Value etc.) or LCV (aka Net Heating value), and whether gross output (at the generator terminals) or net output (at the power station fence) are being considered. The two are separate but both must be stated. Failure to do so causes endless confusion.

Related, more specific terms include

Fuel heating values and efficiency[edit]

In Europe the usable energy content of fuel is typically calculated using the lower heating value (LHV) of that fuel, which definition assumes that the water vapor produced during fuel combustion (oxidation), remains gaseous, and is not condensed to liquid water so the latent heat of vaporization of that water is not usable. Using the LHV, a condensing boiler can achieve a "heating efficiency" in excess of 100% (this does not violate the first law of thermodynamics as long as the LHV convention is understood, but does cause confusion). This is because the apparatus recovers part of the heat of vaporization, which is not included in the definition of the lower heating value of fuel[citation needed]. In the U.S. and elsewhere, the higher heating value (HHV) is used, which includes the latent heat for condensing the water vapor, and thus the thermodynamic maximum of 100% efficiency cannot be exceeded with HHV's use.

Example of energy conversion efficiency[edit]

Conversion process Conversion type Energy efficiency
Electricity generation
Gas turbine Chemical to electrical up to 40%
Gas turbine plus steam turbine (combined cycle) Chemical/thermal to electrical up to 60%
Water turbine Gravitational to electrical up to 90% (practically achieved)
Wind turbine Kinetic to electrical up to 59% (theoretical limit)
Solar cell Radiative to electrical 6–40% (technology-dependent, 15-20% most often, 85–90% theoretical limit)
Fuel cell Chemical to electrical up to 85%
World Electricity generation 2008 Gross output 39% Net output 33%[1]
Combustion engine Chemical to kinetic 10–50%[2]
Electric motor Electrical to kinetic 70–99.99% (> 200 W); 50–90% (10–200 W); 30–60% (< 10 W)
Natural process
Photosynthesis Radiative to chemical up to 6%[3]
Muscle Chemical to kinetic 14–27%
Household refrigerator Electrical to thermal low-end systems ~ 20%; high-end systems ~ 40–50%
Incandescent light bulb Electrical to radiative 0.7–5.1%,[4] 5–10%[citation needed]
Light-emitting diode (LED) Electrical to radiative 4.2–14.9%,
Fluorescent lamp Electrical to radiative 8.0–15.6%,[4] 28%[5]
Low-pressure sodium lamp Electrical to radiative 15.0–29.0%,[4] 40.5%[5]
Metal-halide lamp Electrical to radiative 9.5–17.0%,[4] 24%[5]
Switched-mode power supply Electrical to electrical currently up to 96% practically
Electric shower Electrical to thermal 90–95% (multiply with the energy efficiency of electricity generation for comparison with other water-heating systems)
Electric heater Electrical to thermal ~100% (essentially all energy is converted into heat, multiply with the energy efficiency of electricity generation for comparison with other heating systems)
Firearm Chemical to kinetic ~30% (.300 Hawk ammunition)
Electrolysis of water Electrical to chemical 50–70% (80–94% theoretical maximum)

See also[edit]


  1. ^ IEC/OECD 2008 Energy Balance for World, accessdate 2011-06-08
  2. ^ "Motivations for Promoting Clean Diesels" (PDF). US Department Of Energy. 2006. [dead link]
  3. ^ Miyamoto K. "Chapter 1 - Biological energy production". Renewable biological systems for alternative sustainable energy production (FAO Agricultural Services Bulletin - 128). Food and Agriculture Organization of the United Nations. Retrieved 2009-01-04. 
  4. ^ a b c d Luminous efficacy#Lighting efficiency
  5. ^ a b c Light Pollution Handbook. Springer. 2004. 

External links[edit]