I think it's a reasonable conclusion that solar heat will not keep a pool at 80 degrees or higher October through March unless the solar panels are at, or close to direct southern exposure. Also, quality companies like Jack's Solar will recommend adding as many panels as there is room on the roof to increase the pool temp. That is very good advice.
I have a pool/spa with gas heat assist. Because Jack put 50% more panels than required on my south roof, (six years ago), my pool stays in the high 80's except in January and February. In those months I use the gas assist for about 20 minutes to heat my spa to 99 degrees, and I use the spa most days. When I shut the spa off that warmer water mixes back into the pool and helps maintain the 83 pool setting. It is very rare when the gas heater kicks on to warm the pool only. That has occurred only when we have had subfreezing temps at night for several days. Before solar heat, I had once had a winter gas bill of more than $500. My highest one since has been about $150. I haver never used a pool cover.
There is another factor as well. The control systems and timer for the solar unit must be designed to maximize the solar heated water. This is another important reason to have an expert like Jack design and build the system.
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