I use 1 50,000 lumen hps for a 2'X2' to 2'X3' area works for me but have been told can never have to much light unless youre burning or bleaching them. I am going to upgrade to 100,000 lumens in a 2'X 3' area. thas what i use and i get pretty good results havent ever figured the grams per watt per square foot formula cause i suck at math. but i get any where between min of 2 oz and max of 3.4 oz per plant of dried bud depending on strain.