# How to convert lux to watts

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 formula

### Lux to watts calculation with area in square feet

So 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)

So 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 1

What is the power consumption with illuminance of 40 lux, luminous efficacy of 15 lumens per watt and surface area of 200 square feet?

P = 0.09290304 × 40 lx × 200 ft2 / 15 lm/W = 49.54 W

#### Example 2

What is the power consumption with illuminance of 60 lux, luminous efficacy of 15 lumens per watt and surface area of 200 square feet?

P = 0.09290304 × 60 lx × 200 ft2 / 15 lm/W = 74.32 W

#### Example 3

What is the power consumption with illuminance of 100 lux, luminous efficacy of 15 lumens per watt and surface area of 200 square feet?

P = 0.09290304 × 100 lx × 200 ft2 / 15 lm/W =123.87 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 1

What is the power consumption with illuminance of 40 lux, luminous efficacy of 15 lumens per watt and surface area of 18 square meters?

P = 40 lx × 18 m2 / 15 lm/W = 48W

#### Example 2

What is the power consumption with illuminance of 60 lux, luminous efficacy of 15 lumens per watt and surface area of 18 square meters?

P = 60 lx × 18 m2 / 15 lm/W = 72W

#### Example 3

What is the power consumption with illuminance of 100 lux, luminous efficacy of 15 lumens per watt and surface area of 18 square meters?

P = 100 lx × 18 m2 / 15 lm/W = 120W

## 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 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

Watts to lux calculation ►