Efficiency is usually the ratio of the energy output to the energy input of a system.
Gordon (1978, p.365) argues against the over-extension of the concept, saying that while it is something reasonably attributable to, say, an engine in specific terms, such as fuel consumption and power output, efficiency has no general meaning. For instance, the noise and smell an engine might make, or the human stress suffered by its operator, will tend to escape consideration. Moreover, many factors will not be quantifiable, or will be incommensurable.
A distinction can be made between efficiency and efficacy, construed as the useful output of a system in an unquantified sense.
UK Department of the Environment, Transport and the Regions Energy Efficiency web-site.
Jesse H. Ausubel - Can Technology Spare the Earth?
American Scientist, March-April 1996.
© Paul Taylor 2001