The source is a best selling author and THE dude the industry pays to consult for them and give talks at all the big conferences. I picked a short video just for ease of linking right to it, but his infographics are used the world over.

Also, if you looked at the video, you would see it also praised wind as the solution, although Germany doesn't have the best profile there it is much better than solar. Germany has paid more for solar than California where it makes sense, for way way way less output.