It depends. I've gigged with 30 watts but it was a Matchless dc30. I've also miked a champ. My Markv boogie can do just about any gig. You just have to experiment but if you don't have time or its a new stage it helps to have an amp that can fit into many situations.
I'd say in general 40 watts is enough but sometimes it isn't. And when it isn't life can suck.