Theman: Thanks for the link.
E-Athlete: Yeah, I believe it is possible to grow a plant to harvest with a 15-watt. It will take a long time though, even if it is lowryder. Your yield/potency, as others have noted, will also suffer.
If you have a plant going, try keeping it in veg with your 15/W until you can afford a cheap HPS system. Use an MH conversion bulb to squeeze some more veg out of the plant, then 12/12 with HPS bulb. Then, after whatever harvest you get, you'll be well positioned for your next grow.
I'm in a similar position as you. Ready to invest in lights, but don't have the particulars of my grow room figured out yet.