Ok you must use a 1000 watt bulb in a 1000 watt ballast no mixing watts of bulbs and ballast unless you want a nice explosion and fire.If you do go ahead and stick a 250 bulb in a 100 watt ballast and plug er in.And no a 15 watt bulb wont work to grow a plant.Is it a cfl or tube florescent? either way you need lots more light looottttsssss more