It depends on the size of the venue, the number of people in it, and how loud your band plays.
Jazz trio into mostly empty bar? 200 watts going into a 2x10, 1x12, or 1x15 should be plenty.
Rock band and the bar is half filled? 300 watts going into a 2x10, 1x12, or 1x15 - but you'll need to cut your lows in order to project your sound thru the mix and to the back of the room.
Rock or metal band into crowded house or guitards involved or you use distortion or you play with lots of lows? 300-500 watts going into dual 2x10, 1x12, 2x15 or a 4x10, 6x10, 8x10 or more.
With any of the above examples, remember that as a gigging musician you don't want to be caught short and get the complaint that the audience couldn't hear you because the rest of the band was playing louder than you could keep up. So remember you can always bring a 4x10 (or two 2x10's) to a gig where you really only needed a 2x10. Just don't turn it up louder than you need to.
Alternately, get any rig you want, use it as a stage monitor only, and run a DI into the PA - assuming your band brings a PA to all its gig because not all bars provide one.