How to convert electric power in watts (W) to illuminance in lux (lx).
You can calculate lux from watts, luminous efficacy and surface area.
Watt and lux units represent different quantities, so you can't convert watts to lux.
- Watts to lux calculation with area in square feet
- Watts to lux calculation with area in square meters
Watts to lux calculation formula
Watts to lux calculation with area in square feet
The luminous flux ΦV in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W):
ΦV(lm) = P(W) × η(lm/W)
The illuminance Ev in lux (lx) is equal to 10.76391 times the luminous flux ΦV in lumens (lm) divided by the surface area A in square feet (ft2):
Ev(lx) = 10.76391 × ΦV(lm) / A(ft2)
So the illuminance Ev in lux (lx) is equal to 10.76391 times the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W) divided by the surface area A in square feet (ft2):
Ev(lx) = 10.76391 × P(W) × η(lm/W) / A(ft2)
So
lux = 10.76391 × watts × (lumens per watt) / (square feet)
or
lx = 10.76391 × W × (lm/W) / ft2
Example
What is the illuminance with power consumption of 60 watts, luminous efficacy of 15 lumens per watt and surface area of 200 square feet?
ΦV = 10.76391 × 60 W × 15 lm/W / 200 ft2 = 48.44 lx
Watts to lux calculation with area in square meters
The luminous flux ΦV in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W):
ΦV(lm) = P(W) × η(lm/W)
The illuminance Ev in lux (lx) is equal to the luminous flux ΦV in lumens (lm) divided by the surface area A in square meters (m2):
Ev(lx) = ΦV(lm) / A(m2)
So the illuminance Ev in lux (lx) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W) divided by the surface area A in square meters (m2):
Ev(lx) = P(W) × η(lm/W) / A(m2)
So
lux = watts × (lumens per watt) / (square meters)
or
lx = W × (lm/W) / m2
Example
What is the illuminance with power consumption of 60 watts, luminous efficacy of 15 lumens per watt and surface area of 18 square meters?
ΦV = 60 W × 15 lm/W / 18 m2 = 50 lx
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).
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