For a 15,000-gallon pool, you will need an AquaPro 77K BTU or higher pool heater. This size heater is suitable for pools up to 15,000 gallons. For larger pools, such as above ground pools over 20,000 gallons, you may need to consider models like the TropiCal 115K BTU pool heater. The larger the BTU of the heater, the more quickly it will heat your pool water, making it more efficient. This means you can run it for shorter periods and save on energy costs. Additionally, a higher BTU pool heater reduces the waiting time before you can enjoy the warm water in your pool. So, make sure to choose the right BTU pool heater based on the size of your pool for optimal heating efficiency.