With no PA support, 1,000 watts. With PA, 500 watts (300 as a bare minimum).
I concur with Munji. Only way 100 watts will cut, is if the drummer is using brushes. 400 watts + 410 minimum for Rock, Blues, or Funk or anything where you want people to dance..IMO, IME.

edit: I hate distortion. So this is for a clean sound.