How to convert luminous flux in lumens (lm) to electric power in watts (W).
You can calculate watts from lumens and luminous efficacy. Lumen and watt units represent different quantities, so you can't convert lumens to watts.
The power P in watts (W) is equal to the luminous flux ΦV in lumens (lm), divided by the luminous efficacy η in lumens per watt (lm/W):
P(W) = ΦV(lm) / η(lm/W)
So
watts = lumens / (lumens per watt)
or
W = lm / (lm/W)
What is the power consumption of a lamp that has luminous flux of 900 lumens and luminous efficacy of 15 lumens per watt (lm/W)?
P = 900 lm / 15 lm/W = 60 W
| Light type | Typical luminous efficacy (lumens/watt) |
|---|---|
| Tungsten incandescent light bulb | 12.5-17.5 lm/W |
| Halogen lamp | 16-24 lm/W |
| Fluorescent lamp | 45-75 lm/W |
| LED lamp | 80-100 lm/W |
| Metal halide lamp | 75-100 lm/W |
| High pressure sodium vapor lamp | 85-150 lm/W |
| Low pressure sodium vapor lamp | 100-200 lm/W |
| Mercury vapor lamp | 35-65 lm/W |
Energy saving lamps have high luminous efficacy (more lumens per watt).
| Lumens | Incandescent light bulb watts |
Fluorescent / LED watts |
|---|---|---|
| 375 lm | 25 W | 6.23 W |
| 600 lm | 40 W | 10 W |
| 900 lm | 60 W | 15 W |
| 1125 lm | 75 W | 18.75 W |
| 1500 lm | 100 W | 25 W |
| 2250 lm | 150 W | 37.5 W |
| 3000 lm | 200 W | 50 W |