A good starting point for determining the BTU for a replacement system is to check the BTU of the old one. Use the tally plate on the old unit to find the BTU. Most manufacturers include this information in their model number (M/N) as a three-digit number after the first five digits.  If the M/N is*****024, the unit is 24,000 BTU. Some tally plates have the BTU rating in direct sight. If you are unsure, then search the M/N with the manufacturer.