Setting aside all those factors, I think it would be doable, but only barely. We use an 18 kW element for 6 bbl boils, and that’s less power than I’d like, but not by a whole lot. I’d say 4-5 kW per barrel of boil volume would be acceptable.
You have me seriously considering putting in a fume hood and maybe going electric at the same time, BTW. It would be nice to be able to brew more than 5 months a year.