How many watts is enough for 840 glasses

Depends on how many "maybes per gallon" you have!

hmm i had a 2nd thought.. is this about yield? ambient co2 and an efficiennt light will get you somewhere around 1.5-2.0 grams per watt - easy math from there. This heavily depends on having a high end / high-efficacy light and growing things in a competent way that maximizes the space you use. Ccould just as easily end up with 1g/watt if not utilizing space well.. or using a cheap inefficient grow light. 1g/watt is about all you can expect out of street lights, i mean hps/cmh, lol.

Something is lost in translation... watts per area is a poor way to size up a light. Efficacy varies, so different amount of watts are needed. Hours of operation are relative too. Read up on "daily light integral" and you should be able to extrapolate what you need.

C’mon guys. Everyone knows it’s half a watt per glass. So I think 420w will do just fine.

I agree with krisis, don't understand the question.

Not understanding this question