15 watts is really low. You could germinate a seed with 15 watts, but not much else. go to a hardware store, get a few 23-26w 6500K daylight Compact Fluorescent lights. (The twisty ones, if you don't know what i mean.)
You can't grow with any old fluorescent tube. It has to be a growing spectrum (5000K will work, 6500K is superior) Make sure the kelvin tempurature (K) is no less than 5000 for vegetating, and no more than 3000 for flowering. (2700K is ideal flowering spectrum)
regular fluorescent shop lights are 41000K (not suitable)
You could also just go get a 150w HPS and ballast, then you'd be set forever.