I have a 40,000 gallon pool in Maryland. We don't have the option of natural gas and so we have a heat pump - a Pentair UltraTemp 120 Heat Pump. It works fine to keep the pool at temperature - which for my family is around 86 or 87 - from May through September. I just leave it set to that constantly during that time. In October when the night temps start getting cooler, the heat pump only gets the pool up to low 80s during the day typically. I still swim, but the rest of my family doesn't at that point (I grew up with an unheated pool in upstate NY!). We do NOT have a solar cover - just too much trouble to put on and take off every day. But if we did, the pool would obviously stay much warmer and we'd probably be able to achieve the high 80s even into October.