How choosing a different year in "Area Solar Radiation" affects the calculation of the radiation? I understand that months change some parameters in the calculation formulas, but what about the year? What's the difference between choosing 2012 or 2013, or some other year?
It depends what part of earth are you located. If you are within the mid and tropical latitudes solar radiation is constant. This means we always get 12 hr of light and 12 hrs of darkness not like the upper and lower latitudes (poles) where solar radiation intensity changes depending of time of the year.