I want to buy a propane gas pool heater for an above ground 18'x33' pool. I like my temperature to be around 80. I get full sun, when there is sun, so most times I can heat my pool from the sun,, but if we have a cool summer,, it is hard to maintain 80f. So, my question is,, how many BTU's do I need to efficiently heat this pool. I thought 130,000 BTU's would be okay, but have been told that I need 200-250,000 BTU. Please help, asap as there are two units on sale, and it ends tomorrow April 18! Thanks