I've been brewing since I started with the propane gas cooktop in our kitchen. The largest burner (where I put my brewpot) is about 8,500 btu. It will boil a full 5 gallons in my 30 qt SS brewpot.....just. I'm talking about a small bubbling boil, none of your rolling volcanic boils. I achieved a somewhat better boil by insulating my SS brew kettle. If you can go 10K btu or larger, I suggest that you do so. I'm moving outdoors shortly to do my first AG brew......quite a step up to 185K btu.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.