I am not sure how much of these 28% are due to the distribution of solar panels in Germany and how much is due to seasonal effects. Solar irradiation varies a lot over the year. I vaguely remember that it goes down to about 300W/m^2 at noon in winter and peaks at around 1kW/m^2 at noon in summer, but I don't have time to verify that right now. The exact numbers vary by location, of course.

But I can confirm that the sun is shining because it's shining directly on my roof and satellite images show a clear sky for anything south of Frankfurt.