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What's the difference between efficiency and efficacy?
Lighting fixture efficiency is the ratio of the total lumens exiting the fixture to the total lumens initially produced by the light source. For example, if a bare 100W incandescent lamp (light bulb) produces 1,000 lumens, and it is put into a lamp fixture that delivers 700 lumens, this would be an example of a 70% efficient fixture.
Efficacy is a term normally used in cases where the input and output units differ. In lighting, we are concerned with the amount of light (in lumens) produced by a certain amount of electricity (in watts). Efficacy = Lumens Per Watt