It's far easier to underdo lighting than overdo it...the real question is, what kind of harvests are you looking to pull? If you're completely content with average yields, then you don't need to go whole hog. And vice versa of course. What is your grow space dimensions? This will also be a large factor in determining how much light you need. If memory serves correctly, I think that you want at least 30-40w per square foot of HID lighting? Someone plz correct me if I'm wrong here!