115 watts per square foot

I have found the best way to measure is lumens per sq foot. I have found that 2500 to 300o lumens per sq ft is optimum. CFL's are great for vegin but u don't get the the great size when flowering under them no matter how much u use HPS is the best way to go 4x4 room 400w HPs (45,000 lumens on a 400w hps 45,000/16 sq ft = 2812 Lm/Sq ft. This is not minimum nor optimum but it gives good coverage and when combined with reflective material gives even better coverage. Also when constructiong a grow box build a box with the lite set on the outside. this will give you extra space for vertical growth any built box should be 5 ft-6ft in height. IMHO.