How to convert lumens to watts

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.

Lumens to watts calculation formula

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)

Example

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

Luminous efficacy table

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 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).

Lumens to watts table

Lumens Incandescent
light bulb
watts
Fluorecent
 / 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

 

Watts to lumens calculation ►

 


See also

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