I'm looking to buy a electric pool heater for my above ground 18' x 48" round rubber pool in my signature. I have looked around a little bit and have not been able to determine how many BTU's are required to raise my water temp 20 degrees like from 70 to 90. Naturally I'm not looking to spend too much money and don't expect to use the device very often but I do hope it can manage to heat my pool over a cold weekend about twice a year. I'd love to see a flat/easy formula too that shows how many BTU's I need. I do expect it to run on a 240 volt circuit.