[Zeke]

I can't do the formula, but the difference in the water temp with 60 gallons of hot water won't be much, IMO. You'd be better off building a wood fire and running some copper through it.

Looking at the BBQ, all the copper and fittings and then the cost of the propane, that looks silly.

Maybe you could score a used on demand unit and use it for the pool.

BTW, we must use a couple of full tanks of hot water daily. That's for 2 adults. If it was 5 bucks a tank, we'd be doing something else. I'm guessing less than a dollar per tank.