remember, it is the cosine of the angle of incident light, however the sun moves all day and rooftops are "fixed angle" (so far, but who knows, maybe someone will invent a flat, non moving tracker)
(trackers give appx 30% gain, or throw on a few more panels, back in time 40 years ago when PV was really expensive)
There are world Solar angle calculators. to give you an idea of roof/panels/etc.
my ~23 degree angle (rise of 5 in run of 12) is best for summer as I am at ~40 degrees N latitude
An easy rule of thumb for appx 40 degree north or south latitude is subtract ~15 for an optimal tilt of around ~25 degrees to max out summer gain
(the earth tilts +/- ~23.5 degrees effectively as it goes around the sun or around ~47 degrees)
(there are LOTS of books and info over the last 40-50 years)
Solar Angle Calculator