Location and coverage percentage are everything when it comes to solar, the problem is in the winter time you are fighting 3 issues at once, lower outdoor temperatures, less hours of sunlight, and less heat available from the sun. However for central FL it is possible to maintain near year around swim temperatures with a combination of a solar cover to prevent evaporative cooling, and around 100% panel to pool surface area size ratio. Exact panel size also depends on how warm you want the water, if panels are at optimal angle, shading, etc.
p.s. For me with about 75% coverage and an indoor pool in south Louisiana I loose the battle against declining temperatures usually in mid November when the first strong cold fronts come through and we have a week of weather that never gets out of the 50's and lows somewhere in the 30's. For me things just seem to spiral down rapidly going from mid 80 degree water one week to high 70's the next, and somewhere in the 60's the next.