The difficulty of coming up with an exact amount is with knowing how open your regulator is when you are maintaining your boil. This will of course be different for burners with different BTU ratings. I believe most standard turkey fryers are 55,000 BTU/hr. at max.
To get a true number you could weigh your propane tank before & after the brew session.
I once did a SWAG estimate with the following assumptions:
- 55,000 BTU/hr rated burner
- Burner on full 100% for 30 min. to heat sparge water (27,500 BTU)
- Burner on full 100% for 30 min. to bring wort up to a boil (27,500 BTU)
- Burner on 30% for 60 min. to maintain the boil (16,500 BTU)
Total BTU for one brew session = 27,500 + 27,500 + 16,500 = 71,500 BTU
A full 20# propane tank will hold 4.7gallons of propane. Each gallon provides 91,690 BTUs.
Therefore, a full tank is 430,943 BTUs and at 71,500 BTU per brew session, you should be able to complete 6 full brewing sessions before your tank runs empty. (If all above assumptions are valid)
Of course if you heat sparge water separately on the stove you'll use less propane per session. Another consideration is that maybe the burner isn't quite running at 100% when you're heating up to a boil. Maybe it's 90% or even a little less. Weather conditions will affect how long it takes to heat the water too.
Like I stated earlier, a true number would come from weighing the tank before & after, then using those numbers to figure an average brew day.
** see "Propane Tank Remainder Measurement" section of the wiki link.
http://en.wikipedia.org/wiki/Propane
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