Luminous efficacy tells you how efficiently a light source turns electricity into light, in lumens per watt.
What the terms mean
| Symbol | Meaning |
|---|---|
| Efficacy | Lumens per watt (lm/W) |
| Φ | Light output, in lumens |
| P | Power draw, in watts |
Worked example
A fixture producing 4,000 lumens on 40 watts has an efficacy of 100 lm/W.
See the LED Efficacy Comparison and Power Formula.
Luminous efficacy explained
Luminous efficacy tells you how efficiently a light source turns electricity into visible light, in lumens per watt (lm/W): efficacy = lumens ÷ watts. The higher the number, the more light per watt — and the lower the running cost and heat. Old incandescent bulbs managed only 10–15 lm/W; halogens a little more; fluorescents 50–100 lm/W; and modern LEDs commonly reach 100–150+ lm/W, with the best laboratory devices higher still.
Efficacy is the single best number for comparing two lamps’ energy performance, but pair it with CRI and color temperature — pushing efficacy to the maximum sometimes trades away color quality. For a real fixture, remember the optic absorbs some light, so delivered (fixture) efficacy is a bit lower than the bare LED’s rated efficacy.