Fred Davis Corporation Newsletter

Terms and Definitions: Efficacy vs Efficiency

In our upcoming posts, we will survey the current status of LED fixtures across different categories. As you read, lighting terminology will be important. Most critical is the difference between lighting efficacy and lighting efficiency, two different measures of output over input.

Efficacy: Efficacy is the ratio of light output (lumens) to energy input (watts). So when one mentions lumens per watt, they are talking about a lamp’s or a fixture’s efficacy.

Efficiency: Efficiency of a fixture is the ratio of the light output from the fixture over the light output of the source (lamps or chips). Since both are measured in lumens, efficiency is calculated as a percentage. The most efficient fixture is a socket with a bare bulb since 100% of the source lumens are emitted.

If we use a term you don’t know, please ask us. You can either write a comment or send us an email at

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