I read that you go by the square foot of the space? I've always used fluoros partly because less heat, i don't have that big of a space and they use less energy. not sure what the lumens rating is on the bulbs. i'll have to check think it's about 1000 or 2000 per, so 4 lights would make it anywhere from 4000 to 8000. they seem to work just fine if you get the right spectrums.