Most of the time, I'm using 1-2w listing to the radio. If I want to rock out, or make the house vibrate I use around 50-60w this is out of a 200w amp. At 100db (system can do well over 100db without clipping) at 60w, there will be 100+w peaks.
Since it takes double the power to produce 3db gain, it's good to have all the power, and use it when you want it. Generally a 200w amp will sound better at 50w, then a 50w amp at 50w. Plus staying that far under the amp's peak power produces much cleaner power to the speakers.
Most of the time, we will all be happy with a 20w amp, but I would never buy anything under 100w