Lighting
Luminaire luminous efficacy
- where
- luminaire efficacy (lm/W)
- luminaire luminous flux (lm)
- power (W)
Explain this equation
Gives how much light a luminaire produces per watt of electricity. Use it to compare fittings on energy efficiency.
Φ is the total luminous flux the complete luminaire emits, in lumens. P is its total electrical input in watts, including the driver or control gear. K is the efficacy in lumens per watt.
It is a simple ratio of output to input. Unlike efficiency it is not a percentage, because lumens and watts measure different things.
4000 lm from 40 W is 100 lm/W. Modern LED luminaires commonly sit around 100 to 150 lm/W, older fluorescent fittings lower, and tungsten lamps only about 10 to 15 lm/W.
Watch out: compare whole-luminaire figures, not LED chip figures, which are higher. High efficacy does not guarantee good light distribution or low glare.
Luminous efficacy measures emitted light per watt of electrical input. Compare complete-luminaire values on the same basis when evaluating products.
A high lm/W value does not guarantee good lighting on the task: distribution, glare, colour quality, controls and maintenance also matter. To recover flux, multiply efficacy by power.
Enter your values to see the result.
Common mistake
Do not confuse luminous efficacy in lm/W with efficiency expressed as a percentage.
Limits
Use total luminaire output and total input, including driver losses. Do not compare LED-chip efficacy directly with complete-fitting efficacy.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.