"Your guitar is too fucking loud!"
Have you ever heard this? Probably, but if you haven't, perhaps someone should have told you.
So how much power do you need? I don't know, but I'm pretty sure it's not 25, 50 or 100 watts. 100 watt amps were made for when there were no good sound reinforcement public address systems like there are now. They WERE the PA. Hell, most Fender designs pre-60's were designed to squeeze as many clean watts as they cheaply could out any given tube compliment. And those tweed amps were meant to be the amp for the whole band - mic's, guitar, bass. Mic'ing drums? Nope?
OK - so practically what is the answer? Depends on how aggressive your drummer is and how clean you like to play. I've played slightly broken up cleans and crunch with fairly loud drummers with a 5 watt amp into an efficient 4x12 cab. Yes, it's loud enough. I played outdoor block parties. I don't even think I mic'ed up. Small bars and coffee shops, not a problem. And you're not so loud that you are overpowering the space. Some of the worst sounding bands I have seen were playing WAY too loud in small bars. They would have sounded 100x better with keeping the volume appropriate for the space. And if you play an auditorium, chances are they are going to have a decent PA. Mic up. Keep the stage volume reasonable. Everyone will play better and your band mates won't hate you or keep telling you to turn down.
So I've found that 5 to 18 watts is a great range. 5 watts is nice if you don't need super clean tones or you play smaller or lower stage volume venues like churches, coffee shops, small bars. 12-15 watts is a pretty good level if you need some clean volume. These amps can get away unmic'ed in pretty big auditoriums though. I'd recommend mic'ing just keep the sound even so you don't get a hot spot where the amp/speaker is and a dead spot on the other side of the stage. Hell, mic up in a bar with a 5 watt amp. You don't have to run much mix through the PA, but the sound will be fuller. PAs are so good and cheap and your singer is going to use one anyway, so take advantage.
So yeah, your guitar is probably too damn loud, and even a 5 watt amp with an efficient speaker cab cranked is pretty damn loud. And a 5 watt amp is technically about 6dB less than a 15-18 watt amp. It's not as much as you might think in terms of volume difference. So anyway, you might still be too loud, but you'll be less loud and you'll still be able to crank the amp an hear the power tubes sing. And if you don't care about that, then you are probably talking to the wrong guy...