its really not about watts at all, its all about the lumens. watts measure energy and power, lumens measure light output. the plants need the light, not the energy running the light. ever notice how you can get equal wattage bulbs that have a different lumen output? check a regular bulb, like a standard incandescent. you can get 60 watt bulbs that have a 100 lumen output, or you can get a 60 watt bulb with a 250 lumen output. obviously these arent suitable for growing whatsoever, but its the same deal with higher output lights.