Propane BTU burner question

This is my first year tapping and I am collecting sap from ten black walnut trees. Although I know black walnuts don't yield as much sap as maples, collection is going well so far. I live on a 20 acre wooded property so my plan was to use wood as my fuel source, however I don't have enough dry wood set aside this spring for boiling the sap so I've decided to use propane. I've found a lot of information here about tanks, burner types, etc. but am not sure how many BTU's the burner should have as I've seen outdoor burners range from 65,000 - 200,000 BTU's. I will be boiling about 20 gallons of sap at a time. I also plan to have two 10 pound tanks of propane on hand to boil the 20 gallons of sap, which I think should be enough. Any thoughts or recommendations are much appreciated!

While it takes BTUs to boil, its more of an issue of surface area that is important.You'll get more evaporation for a wide and shallow pan than you will a turkey fryer pot. I have a double burner camp chef propane cooker that I use to finish syrup that comes off my 2 foot by 4 foot evaporator pan. I had a pan built to fit the cooker, and it works well to finish on, but I'd go broke in a hurry if I used it to cook raw sap to finish. That said, if its just 20 gallons, you should be fine.