Four foot T5 high output fluorescent lights are the best grow light on the market today. Their output is relatively stable over the life of the lamps (20,000 hrs.). The light output of 4 foot T5 high output (54 watt) lamps is much greater than T12 lamps (shop lights), a 4 lamp T5 system rivals 400 watt HID system with way less heat. T12 lamps loose a lot of their intensity after 50% life and should be changed at about half life when used for grow light.