How much light does a 400 watt HPS produce?

It is possible to calculate watt from lumens and efficacy.You can't convert watt to lumen because they represent different quantities.

The power P in watt is equal to the lm in Watt, divided by the efficacy of the light.

What is the power consumption of a lamp that has a bright light output of 900lm and a light efficacy of 15lm/W?

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