30-40Watts maybe a little more depending on the size of the venue and how loud the bar itself is.
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Depends on the bar, the amp, and the crowd. In other words there's too many variables.

Generally you're going to want at least 50 watts to be heard in almost any situation, but you can get by with a much lower wattage amp in a lot of situations, especially if you have a microphone.

Wattage does not imply loudness. There are some 50 watt amps that are much louder than 100 watt amps, and some 18 watt amps that are louder than 50 watt amps.

If you can get your amp mic'd up, you don't have to worry about wattage at all.
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Everyone is entitled to an opinion.

Feel free to express yours so I can make an informed judgement about how stupid you are.
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ok that was fun, you can close thread now, thanks for replying
It depends on far too many things (as others have said)
I've gigged with my 5 watt fender vibro champ without a pa (just a 2x12 cab) and it carried fine. But I wouldn't suggest everyone do that
some of the helpful replies are not helpful.

i consistently play 250person clubs/gigs with 20watt marshall never has it been mic'd, and never has it been turned up past 3 on volume (non master marshall). but it depends on music you play. are you needing preamp gain or.....??? if you are just using a single channel amp and using pedals for your gain or other sounds dependnig on the head room you need. way too many circumstances
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50 would be enough.

best have more and not need it than less and need it.
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It depends if its tube or solid state. You could get by at a medium sized bar venue with a 15W tube 1x12 combo. Then just get a mic. Most amps sound better the more you turn them up, so get a small amp that can do the job for pretty much every venue you play, then get a mic for bigger venues and just use your amp and a monitor and let the PA do the work.