How many BTUs does my burner need?

How many BTUs does my burner need?

The other day I made beer with my new 25 gallon pot and I confirmed what I suspected:  A 55,000 BTU burner cannot boil 15 gallons of wort in any reasonable amount of time.  My brewing day was cold and windy, so I never got the wort above 200°F (93°C).  The question I...