I use an 1800W one for 3gal full boil batches. It will boil 4gal, though it's not vicious. It might boil 5gal, but it'll take a while to get there. I just can't see it being able to boil 6gal.
Go to the electric section and see the induction thread, lots of folks are using induction - some brewing 10gal, 5gal, or like me at 3 or less.
Primaries: BECAN Lager (Coffee Blond)
Kegged: CEDAR (American Black Wheat), Session Coffee n' Cream Stout, PILZN (HG Hallertau)
On Deck: All Brews Subject to Change With No Reason - loving small batch brewing, session beers, AND Kegging!