I'm rather new at this so bear with me!......My grandfather just gave me his 4 level grow light rack. Each fixture contains 2 18" 15W T8 tubes. He has been gardening for 60 years so I'm sure he knows what he's doing but 15W seems rather low to me. Is 30W (2 15s) sufficient to grow new seedlings or keep already established plants alive in the winter? Do they make 18" T8 tubes larger than 15W? I had trouble finding any. I can't replace the T8 tubes with CFLs or some other sort of higher intensity light given the fixture correct? Any recommendations/comments/guidance would be greatly appreciated. Thanks!