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 *Φ*_{V}
in lumens (lm) is equal to 0.09290304 times the illuminance *E*_{v} in lux (lx) times the surface area *A* in square feet (ft^{2}):

*Φ*_{V(lm)} =
0.09290304* *×*
E*_{v(lx)}* *×*
A*_{(ft}2_{)}

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

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

So the power P in watts (W) is equal to the 0.09290304 times the illuminance *E*_{v} in
lux (lx) times the surface area *A* in square feet (ft^{2}), divided by the luminous
efficacy * η* in lumens per watt (lm/W):

*P*_{(W)} = 0.09290304* *×*
E*_{v(lx)}* *×*
A*_{(ft}2_{)} /* η*_{(lm/W)}

So

watts = 0.09290304* *×*
*lux* *× (square feet) / (lumens per watt)

or

W = 0.09290304* *×*
*lx* *× ft^{2} / (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 ft^{2} / 15 lm/W = 61.94 W

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

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

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

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

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

So the power P in watts (W) is equal to the illuminance *E*_{v} in lux (lx) times the surface area *A* in square meters (m^{2}), divided by the luminous
efficacy * η* in lumens per watt (lm/W):

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

So

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

or

W = lx* *× m^{2} / (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 m^{2} / 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 | 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 |

## Lux definition

The lux (symbol: lx) is the SI derived unit of illuminance, measuring luminous flux per unit area.[1][2] It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used as both the singular and plural form.

The word is derived from the Latin word for "light", lux.

Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area when the luminous flux is held constant.

One lux is equal to one lumen per square metre:

1 lx = 1 lm/m^{2} = 1 cd · sr / m^{2}.

A flux of 1000 lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux. However, the same 1000 lumens spread out over 10 square metres produces a dimmer illuminance of only 100 lux.

Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same level of lux requires a greater number of lumens.

As with other SI units, SI prefixes can be used, for example a kilolux (klx) is 1000 lux.

*Source: Wikipedia*

## Watt definition

Watt is the unit of power (symbol: W).

The watt unit is named after James Watt, the inventor of the steam engine.

One watt is defined as the energy consumption rate of one joule per second.

1W = 1J / 1s

One watt is also defined as the current flow of one ampere with voltage of one volt.

1W = 1V × 1A

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