Please listen to LucasMan here and not some kid who doesn't have the experience needed.
What most kids tend to think is that a 50 watt amp is so much more powerful than a 30 watt, and that a 100 watt is just insane, right? The truth is that a 100 watt amp is just marginally louder than a 50 watt amp - you will need something like 10 times the power to double the perceived loudness.
Like Lucas says, a 30 watt amp is usually more than enough to cover it in most settings, at least if the scene is not a huge one and/or you have really loud drums and bass and inferior monitoring. I have played large outdoor gigs with a 30 watt amp (a good Vox tube amp, and with great monitors, but still).
What you also need to realize is that many of these huge stacks you see on the large stages is just as much for visuals as for the sound. Many of these bands could do exactly the same with one or two small amps being miked and run through the PA.
Furthermore, when the stage volume goes up because you have an "overkill" guitar player, you being to create problems for the people doing the stage and public sound. Turn down or use a smaller amp and the people running the sound will thank you for it.
Let us not forget that a smaller tube amp turned up sounds way better than a huge stack turned down...
As long as you hear yourself properly and the other band members her you (this is very important for bass, drums and rhythm guitar), than the size of the amp doesn't matter. The out (public) volume is handled by the PA, and the stage is covered by monitors - side-fill, wedges and/or in-ear.