As you can tell from the previous posts, there isn't a definite rule to go by. Here goes with my experience:
Many years ago I bought a small PA, just a 4 input, 150 watt head with a couple of 12 inch 2 way speakers. Worked well for practices, and I've used it or loaned it out for use in gigs. One night I listened to another band, just running their lead vocalist through this setup (no monitors), and the vocals were loud. Loud as in shrill, uncomfortably so. But for smaller coffee-house type gigs, where the volume could be turned down, it would work great, even without a monitor (though a single 5 inch hot spot would be nice).
Fast forward to my current PA setup which I've been using for the last couple of years, running sound for a 6 piece party band. A pair of 15 inch, 3 way main speakers each getting 150 watts; two pairs of monitors, each monitor speaker getting 75 watts. Five vocal mics, plus acoustic guitar, sometimes a little lead guitar or bass, occassionally cymbals or hihats in the mix. Typical crowds of 100 to 200 people. The volume is good, not overpowering, but acceptable. Yet I'm completely out of headroom. That's why tomorrow the brown truck should be delivering MORE POWER. Each main speaker will now get 350 watts, each monitor speaker will get 150 watts. Should be much better. Total system power will be 1300 watts.
It would be tempting to say "buy as much PA as you can afford" but there are other factors. PA gear costs money. This stuff is very heavy, I can no longer set up by myself (I can't lift a 100 lb speaker and set it on top of a speaker stand by myself without getting a hernia). The gear completely fills the back of a pickup as it is--what next, buy a van or a trailer? And electrical power is sometimes a problem, there's nothing like tripping a breaker in the middle of a gig. Furthermore, it takes a couple of hours to set up as it is, add more stuff, the time factor increases. And the more stuff there is, the more likely something will be lost/broken/stolen.
PS these watts are real RMS. Not "program" or "peak" -- those inflated ratings are useless.