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)
watts = lumens / (lumens per watt)
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
|Tungsten incandescent light bulb||12.5-17.5 lm/W|
|Halogen lamp||16-24 lm/W|
|Fluorescent lamp||45-75 lm/W|
|LED lamp||30-90 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).
|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|