Your assumed roof areas seem a bit small. Mind you, I am assuming that if you go to the trouble of doing this, you go to the trouble of building the house to match, so that the roof is braced to take the extra load, and you might as well at that point is the classic American saltbox house design, in which the roof peak is at one edge, not the center, so that you can use the entire roof. A few million homes over 20 years appears to refer only to new home construction. Even if you do not use saltboxes, 25-30 m^2 is only 250-300 square feet of receptor, which seems rather small.

Some would suggest that relocating the solar panels to Algeria and laying cables would work better.

We've talked about the Algeria option (which makes much more sense) here before:

http://anz.theoildrum.com/node/3791

With regards to the article, why were figures for northern France rather than southern France used ? Surely PV would go mainly towards the Mediterranean coast, where the hours of sunligh are longer and the periods of cloud far fewer ?