How can you tell if a propane burner is large enough for boil?

I did a test run on my electric cook top. 3.5 gallons boiled in 30-40 minutes. I should've checked the temps, but forgot. So, looks like I might be able to do a few partial boils and save the cash by using my stove. Thanks for the help guys!

In addition to the answers above, make sure that there's an adequate flow control mechanism included, such as being able to fine tune the flame by adjusting the O2. If there's isn't enough granularity of control, it will lead to more boil overs.

If you think about what the turkey fryer is meant to do, heat up a few gallons of oil to around 300° F, then you should reach the conclusion that it will be fine for boiling water.

In a nutshell: 30,000 to 50,000 BTUs will heat up your five gallons with ease.

A.J. deLange over at the Homebrew Digest #5092

Temperature shifts in water are very easily calculated from heat inputs because 1 BTU is the amount of heat required to raise 1 pound of water 1 degree Fahrenheit. .... The other piece of essential information is that a gallon of water weighs about 8.3 pounds at room temperature (but only about 8 pounds near boiling). Thus the amount of time (hours) to raise a given volume of water from x to y degrees is 8.3 times the number of gallons times (y-x) divided by the heat input in BTU per hour.

My emphasis added.

He goes on to say:

The tricky part is getting the heat input. With a pot over a burner a lot of the heat is lost. The best way to get useful data is to put a measured amount of water into the vessel, measure the temperature, turn on the heat for a specific length of time, say 1/4 hour, and measure the temperature rise. The temperature rise multiplied by the mass of the water (8.3 times the number of gallons) is the number of BTU the burner transferred to the water in the vessel in 1/4 hour. Four times that is the BTU per hour value which should be used in future calculations.

The #1 issue with propane cookers is that the BTU ratings aren't always accurate. I talked with a heating engineer once about this. Apparently, there are no solid industry standards when rating propane cookers. A BTU is a BTU true, but the way you measure it can take different forms. So when looking at burners you can't really compare between manufactures. What may be one companies 50K BTU is anothers 30K BTU, and you'll get the same performance out of each. BUt you may spend more to get the 50K thinking you are getting more power.

The good news is that you CAN make comparisons within a manufacturer. So if you look at one manufacture who has a 50K, 100K and 150K burner. You can feel confident that they are each more pwerfull than the one below it. Because the company uses the same test method for all its burners.

In general, I'd suggest looking for burners that are 1) used by many brewers and 2) look for ones that have 50K or better BTU ratings.

I use a bayou classic and I am pretty please with it. I also have a canjun cooker that works just a little bit better. I find that the bayou 'banjo' style puts out a lot of heat, but the air intake could work a little better at high out put. The canjun style seems to work great at all ranges of output, albeit a little less powerful than the bayou.

In general the batches you will brew will be about 5 to 6 gallons. That means you'd better use a 6 or 7-gallon pot so that there is some extra space and you avoid boil-overs. If a propane burner is large enough to carry a 7-gallon pot, then it's also powerful enough to get a 6-gallon wort boiling. So get the pot, check its diameter and use a propane burner where the pot fits. :)