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 *Φ*_{V} in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy * η* in lumens per watt (lm/W):

*Φ*_{V(lm)} = *P*_{(W)} ×* η*_{(lm/W)}

The illuminance *E*_{v} in lux (lx) is equal to 10.76391 times the luminous flux *Φ*_{V} in lumens (lm) divided by the surface area *A* in square feet (ft^{2}):

*E*_{v(lx)} = 10.76391 ×* Φ*_{V(lm)} /* A*_{(ft}2_{)}

So the illuminance *E*_{v} 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 (ft^{2}):

*E*_{v(lx)} = 10.76391 ×* P*_{(W)}* *×* η*_{(lm/W)}* */* A*_{(ft}2_{)}

So

lux = 10.76391 ×* *watts × (lumens per watt) / (square feet)

or

lx = 10.76391 ×* *W × (lm/W) / ft^{2}

#### 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?

*Φ*_{V} = 10.76391 ×* *60 W × 15 lm/W / 200 ft^{2} = 48.44 lx

### Watts to lux calculation with area in square meters

The luminous flux *Φ*_{V} in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy * η* in lumens per watt (lm/W):

*Φ*_{V(lm)} = *P*_{(W)}* *×* η*_{(lm/W)}

The illuminance *E*_{v} in lux (lx) is equal to the luminous flux *Φ*_{V} in lumens (lm) divided by the surface area *A* in square meters (m^{2}):

*E*_{v(lx)} = * Φ*_{V(lm)} /* A*_{(m}2_{)}

So the illuminance *E*_{v} 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 (m^{2}):

*E*_{v(lx)} = * P*_{(W)}* *×* η*_{(lm/W)}* */* A*_{(m}2_{)}

So

lux = watts × (lumens per watt) / (square meters)

or

lx = W × (lm/W) / m^{2}

#### 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?

*Φ*_{V} = 60 W × 15 lm/W / 18 m^{2} = 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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