How to convert illuminance in lux (lx) to electric power in watts (W).
You can calculate watts from lux, luminous efficacy and surface area.
Lux and watt units represent different quantities, so you can't convert lux to watts.
- Lux to watts calculation with area in square feet
- Lux to watts calculation with area in square meters
Lux to watts calculation formula
Lux to watts calculation with area in square feet
The luminous flux ΦV in lumens (lm) is equal to 0.09290304 times the illuminance Ev in lux (lx) times the surface area A in square feet (ft2):
ΦV(lm) = 0.09290304 × Ev(lx) × A(ft2)
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 the power P in watts (W) is equal to the 0.09290304 times the illuminance Ev in lux (lx) times the surface area A in square feet (ft2), divided by the luminous efficacy η in lumens per watt (lm/W):
P(W) = 0.09290304 × Ev(lx) × A(ft2) / η(lm/W)
So
watts = 0.09290304 × lux × (square feet) / (lumens per watt)
or
W = 0.09290304 × lx × ft2 / (lm/W)
Example
What is the power consumption with illuminance of 50 lux, luminous efficacy of 15 lumens per watt and surface area of 200 square feet?
P = 0.09290304 × 50 lx × 200 ft2 / 15 lm/W = 61.94 W
Lux to watts calculation with area in square meters
The luminous flux ΦV in lumens (lm) is equal to the illuminance Ev in lux (lx) times the surface area A in square meters (m2):
ΦV(lm) = Ev(lx) × A(m2)
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 the power P in watts (W) is equal to the illuminance Ev in lux (lx) times the surface area A in square meters (m2), divided by the luminous efficacy η in lumens per watt (lm/W):
P(W) = Ev(lx) × A(m2) / η(lm/W)
So
watts = lux × (square meters) / (lumens per watt)
or
W = lx × m2 / (lm/W)
Example
What is the power consumption with illuminance of 50 lux, luminous efficacy of 15 lumens per watt and surface area of 18 square meters?
P = 50 lx × 18 m2 / 15 lm/W = 60W
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 |
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