Maybe late to the conversation, but just to share the newbie rule of thumb I heard, is that the volume you generate goes up about 10% every time you double the wattage of the amps (200 watts will give you 110% the volume of 100 watts) but goes up proportionate with the surface area of your speakers (e.g. a 2x12 cabinet will be twice as loud as a 1x12 with the same same amount of power driving it). Extra wattage doesn't get you loudness so much as headroom; if you're concerned about being loud enough, add more speakers.
In my experience, basically rock cover band bar gigs, I've done fine with 200 watts into either a 1x15 or 2x10. And if there's enough PA to do the heavy lifting, you don't even need that.