As you may have gathered. There is no answer to your question. Way too many variables involved. I've never played live without at least 40 watt Super or Mesa and most of the time, I didn't need it all. And there was a time or two I played with two 40 watts and had one of those mic'd. You have the venue question; the music question; the drummer question; the PA question; the audience question; the acoustics question; the effects pedal question; the clean/dirty question and a lot more. So, take all of this, put it in a pot and stir on simmer. Then, go pick out an amp you like the sound of and can afford and take it home and enjoy it. There is no rule in life that you can't buy another amp sometime in the future. If, though, you are going to only buy one amp and only one amp that you know will be used for playing live at unknown venues/gigs, I'd probably tell you to overbuy rather than underbuy. You can always get your sound from pedals, playing through a clean channel on the amp at lower volume (which I always do because of the number of different sounds I needed for cover songs), but you cannot increase the volume of an amp beyond what it will put out and will then have to rely on whatever PA you have available. Now, I personally like mic'ing my amp and playing through a PA whenever the system will support it. But at most of the places I played, there was no decent PA available and our band's PA simply wouldn't support two guitars, vocals and drums worth a flip. So the answer may be 5 watts one night; 15 another; 25 another and 40 another. If you have a great PA, that answer can be narrowed down to whatever you want the answer to be.