I hate to tell you this, but 15 watt cfl's are next to worthless. Your gonna need between 10-20 of em'.
But don't get discouraged, many good growers start off trying out cheap ineffective light sources, myself included. Trial and error is the best way to test your limits. Shit, my first couple indoor grows were with cheap flouros, I had no where near enough light, then my next couple indoor grows I had to much light (yes it is possible to have too much light, if anyone dosent believe me put a 1000w HPS in a closet thats 2' x 4' x 8'). Now I got it down to a fine art, and with patience you can too.
peace