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The relationship between potentially usable energy and actually used energy. Under the first law of thermodynamics, efficiency is the ratio of work or energy output to work or energy input, and cannot exceed 100 percent. Efficiency under the second law of thermodynamics is determined by the ratio of the theoretical minimum energy that is required to accomplish a task relative to the energy actually consumed to accomplish the task. Generally, the measured efficiency of a device, as defined by the first law, will be higher than that defined by the second law.

Examples: conventional light bulbs transform about 3-4% of the used energy into light; solar cells reach efficiencies of about 11–17% when converting light into electricity; and a solar heating system can transform 25-40% of solar radiation into usable heat.

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