How to convert luminous flux in lumens (lm) to electric power in watts (W).

You can calculate watts from lumens and luminous efficacy. Lumen and watt units represent different quantities, so you can't convert lumens to watts.

## Lumens to watts calculation formula

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

watts = lumens / (lumens per watt)

or

W = lm / (lm/W)

#### Example

What is the power consumption of a lamp that has luminous flux of 900 lumens and luminous efficacy of 15 lumens per watt (lm/W)?

*P* = 900 lm / 15 lm/W = 60 W

## 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 |

Energy saving lamps have high luminous efficacy (more lumens per watt).

## Lumens to watts table

Lumens | Incandescent light bulb watts | Fluorescent/LED watts |
---|---|---|

375 lm | 25 W | 6.23 W |

600 lm | 40 W | 10 W |

900 lm | 60 W | 15 W |

1125 lm | 75 W | 18.75 W |

1500 lm | 100 W | 25 W |

2250 lm | 150 W | 37.5 W |

3000 lm | 200 W | 50 W |

## Lumen unit definition

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time. Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square metre.

If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen (1 cd·1 sr = 1 lm). Alternatively, an isotropic one-candela light-source emits a total luminous flux of exactly 4π lumens. If the source were partly covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.

The lumen can be thought of casually as a measure of the total amount of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.

The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2.

A source radiating a power of one watt of light in the color for which the eye is most efficient (a wavelength of 555 nm, in the green region of the optical spectrum) has luminous flux of 683 lumens. So a lumen represents at least 1/683 watts of visible light power, depending on the spectral distribution.

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