I meant to say this too, cant put too much emphasis on that. If you just plan on playing in your bedroom, if you go too big with it you will shake your walls, piss off your neighbors, and give yourself tinnitus every time you play a song. Not that a single one of those things is bad, but it may not be the most desirable outcome depending on what your looking for. A 15 watt solid state amp will not get loud at all. A 15 watt tube amp (like the Orange Tiny Terror) WILL be pretty loud for bedroom practice. And in all likelihood, if you are playing a gig that a 15 watt tube amp isn't loud enough for, it will probably be mic'd anyway.
That being said, loud is still awesome. I have a Mesa Boogie Mark IV, which is 85 watts (still not THAT high of a wattage compared to whats out there), and I have played unmic'd gigs with some pretty high gain levels with absolutely no problems. Most of the shows I have played with it have been mic'd though and the volume on the amp itself is usually kept pretty low. When I'm just playing in my bedroom, if I put the volume at like 3 it gets to the point of too loud, my dog gets scared. But I love it.