I have a converted keg (15.5 gal) that I routinely do 12 gallon batches in. For this kettle, I get about 1.5 gallons per hour of boil off. Starting with a 13.5 gallon pre boil volume I do have to watch through the hot break, but after that, there isn't a problem at all. If you ran across a converted keg, grab it. If you have a 15 gal and a 20 gal to choose from, get the 20 so that you can brew with a little more margin for error. Plus, if you do a really hoppy IPA, those extra hops will really suck up some volume and you might want to bump up your batch size to compensate.