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Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, 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.

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.

Related, more specific terms include

• Electrical efficiency, useful power output per electrical power consumed;
• Mechanical efficiency, where one form of mechanical energy (e.g. potential energy of water) is converted to mechanical energy (work);
• Thermal efficiency or Fuel efficiency, useful heat and/or work output per input energy such as the fuel consumed;
• 'Total efficiency', e.g., for cogeneration, useful electric power and heat output per fuel energy consumed. Same as the thermal efficiency.