Just curious... what is the approximate heating and cooling degree days where you live? https://www.eia.gov/energyexplained/units-and-calculators/degree-days.php
Perhaps equally important are the average peak solar hours per month. I used http://tsi.tyconsystems.com/html/nrel_lookup.htm when I was studying up before buying my solar system. I count this as one of the estimates I did that was pretty accurate.