Best bet is at close to 0deg but not 0 deg. Slope it 10 deg so the rain does not sit on the panel. Any rack you use can be ballasted by some cement pads without weighing down the roof too much and 10deg doesn't present much wind load.
Point is, you will get much more light and power in the summer than you ever will in the winter so there is no point in trying to optimize for winter use.
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