Yea, my bad. Meant to say 15. Cool. Than I'm squared away for a while. I had one full for a grill I ended up getting rid of and they had a sale when I bought my turkey fryer where you got a free full tank. So I've got 2
I would say between 3 and 5 depending on boil times, I also heat my strike and sparge water on the same burner. I will be switching to an electric heating element soon because at 18 bucks a refill the propane is getting expensive.
Do the math. There is 90k btu's in a gallon of propane. So if your burner is rated at 90k btu than it will burn one gallon in one hour. Hope this helps. By the way there is approx 5 gallons in a 20lb tank.
I've burned 3.5 - 4# per 5 gallon batch (60 min mash and boil) this season. As was mentioned, that number will go up as the temp falls.
As long as I have 4 or more pounds in the tank (tank is 16.6#, so ideally 21# or more), I'll fire it up and I haven't run out yet! (knock on wood )