From what we've seen here, heat pumps are GREAT for heating pools in the spring and summer, being very efficient when the air temp is in the 50's or higher, especially in the 70's and up. But for spas in colder weather? Ben's certainly right. My neighbors with Natural Gas heaters don't use them much unless they are having a pool party because their gas bill goes through the roof!

Solar works well here in the NE, but is inconsistent. 4 or 5 rainy days and your pool temp will drop back into the 70's. A solar cover will help slow that down and hold the heat in at night. But it's a trade-off. I've been running solar panel heating in NJ for 13 seasons now, first with my little 15' Intex donut and now with my FantaSea. Personally, I've been very happy with it. We find we don't use the pool that much in May or September except on hot days.