Okay - here is the thing. I have two 15-gallon planted freshwater tanks and I don't realy need 3.66 watts/gal, 2.4 watts/gal would be just fine. I plan on getting a 2 bulb unit (1 for eash tank). I like the longer reflector thar comes with the 55watt units but don't want that much lighting. I was tring to plan for the future, with the 55watt ballast if I could run 36 watt bulbs I would be able to reuse this in a 55 watt configuration later.