I was more fishing for the
kind of heat source, gas, electric (open) coils or electric flat top.
Mostly in respect to possible ways to insulate those pot(s).
With gas, open flames, using any kind of insulation around the pots or kettle is totally out of the question. They're a fire hazard. Don't!!!
In my old home, on the largest (open) coil I could boil 4 gallons of water without a problem in a 5 gallon enamel canning pot/kettle.
Possibly even a larger kettle/volume, but safety may have become a concern due to construction and stability of those kind of stoves and elements.
In a leased townhome, after that, the brand new flat glass top electric stove that was in there could not comfortably boil 7 gallons of water/wort without some insulation. I used thick, tripled-up towels held with super tight bulldog clips and bungees cords around the kettle,
and leaving the lid on part ways with a thick tripled up towel on top.
I tried straddling the kettle over 2 of the larger burners, but due to the space in between that was no better solution, possibly worse.
When we bought our new house, we bought a flat glass top stove having a huge triple element, especially with brewing in mind. I was hoping it could boil 7 gallons (full volume, all grain). That did not pan out either as the element keeps cycling on and off.
Again, only with insulation (triple layer of Reflectix) around the kettle, the lid on part ways, and again a thick towel on top, I could maintain a mere simmer and still boiled off a gallon an hour. But the sheer weight of that full kettle on the glass top also became a major point of concern.
Inspired by a thread here on HBT, that's when I bought the Avantco IC3500 countertop induction plate, and stick a box fan in the (half) open window behind it. I never looked back. Better yet, I even bought a 2nd plate.
Running a stove fan also increases evaporation and removes heat, which may be part of the under-capacity heating problem, but you really can't do without it, unless you get inventive. But you can condense wort as much as you like, even if it takes 2 or 3 hours. Or use a couple pounds more malt, so you don't have to sparge and later condense as much.
Anything that spills onto your burner is likely to cause smoking or burning. Don't fill pots too high.
In the short run, I'd say split heating/boiling your water and wort over as many burners and pots you have. The larger one(s) holding a bigger pot/volume than the smaller ones.
Even chilling in the sink or a tub with water is more efficient with multiple vessels. Combine at the end.
I bought two 9 quart stainless pots at IKEA for $20 a piece. They're induction ready, and use them all the time. I wish I had bought a few more...
