I struggled with heat pump information and what to believe when building my pool. The truth is, in my case, if the temps stay above 65 during the day and 50 at night, I can heat the pool up to 90 degrees and if I cover it at night, not lose more than 2 degrees. This is with the heat pump running 24 hours a day. My electric rates are pretty low (.07) so it doesn't cost too much (~5$/day). Throughout the month of November, I was able to keep my pool over 85, except for a few days. It worked out for me to be about 1 degree every 3-4 hours, so 6-8 degrees a day with the heat pump running 24 hours. It actually shut off at 90 one day. I also live in an area with fairly high average relative humidity and this helps.
BUT... this is with temps OVER 65/day and 50/night. If the temps stay below 50 at night for a few days, then you will lose any heat you were able to put in, because the heat pump won't run and even with the pool covered at night, the low temps will cause you to lose 4-5 degrees each night, and you can't make up more than 3-4 degrees during the day (12 hours daylight = 3-4 degrees heat for me). It did take about 5-6 days to get from 60 degree water to 85 degree water, due to some nights with low temps where the heat pump would not run.
As always, your mileage may vary. But consistent temps and high relative humidity will yield good results.
I went with a Heat Siphon Z575 for a True L 42x16x24, 800 sq ft, approx 28,000 gal (sloped sides). I cover my pool with two 20x25 Home Depot 6 mil vinyl translucent tarps.
If you look a the Heat Siphon charts, you will see that the Z575 actually has a higher COP at the lower BTU's which is where you will be operating during the winter months. But the Z700 will put out more BTU's at that lower temp so it may be a better choice. I'm happy with my Z575.
If its not a violation of forum rules, I can say I was helped by an on-line vendor during my search and they answered all my questions, gave me a great deal and let me put down a REFUNDABLE deposit for a year.
PoolHeatPumps.com
They will answer your calls/emails 7 days a week and will stand behind the product they sell you. Marcus will email you a fairly comprehensive opinion of what size/model he would recommend if you ask him.
No, I don't work for them, but I can say they gave me STELLAR customer service and I believe that deserves me mentioning them.
Sorry for the long post, but I really appreciated the real world performance advice when I was looking so I thought I'd put my .02 in.