In your 4,060 square foot pool, keeping your pool water 10 degrees above ambient air temperature would require you to add 400,000 BTU of heat energy per hour. Divide that by the 91,000 BTU in a gallon of propane means you must burn 4.3 gallons of propane per hour to keep your pool warm. If you wanted to keep your pool 20 degrees above the ambient air temperature, you would burn 8.6 gallons of propane per hour to add 800,000 BTU of energy to the water.
You'll need a big propane tank. Residential propane tanks come in sizes of 250 gallons, 500 gallons and 1,000 gallons. However, local codes may restrict the size tank you can install. If you ran your pool heater 8 hours a day to keep the water in your 4,060 square foot pool 10 degrees warmer than the air, you would use 34 gallons of propane per day. A 250 gallon tank would last you a week, a 500 gallon tank would give you two weeks and a 1,000 gallon tank would last you a month. You would need 68 gallons per day to keep water temperature 20 degrees warmer than the air. At that rate, the 1,000 gallon tank would last you only two weeks..
You can reduce your fuel consumption and the size of your tank by using an insulated pool cover. Evaporation of water is the major cause of heat loss. To keep pace with evaporation heat losses, you must add 10.5 BTU of heat energy per square foot of surface area per hour for each degree above air temperature you want to keep your water.
Cover Cuts Fuel
You can reduce the 400,000 BTU of heat energy needed to maintain water 10 degrees above air temperature by an average 60 percent--to 160,000 BTU per hour-- by keeping your pool covered when you are not actually swimming. Your pool is very long, so you probably would want a manual pool cover reel or motor-driven automatic pool cover.