I have 4 bulbs, each one overdriven 2x over my 55. I'm running two 6500K Phillips Daylights and two 5000K GE Sunshines. The light is intense enough to start my hygro difformis and val. americana pearling in under five minutes. I never really worry about the actual intensity when I see results like that so quickly. Halides may be more intense and have that sweet shimmer going on, T5s may be ultra-efficient, but you can't beat overdriven T8s for initial and maintenance costs over a 4' tank.