I am guessing, covering the roof would not be enough. A large solar panel is barely enough to heat your water. I am just speaking from hear say. My brother is an architect and a huge supporter of wind and solar power, but he has told me that until technology improves, it is not always worth your while, unless you can afford the up front price.
In some countries their is grants for people to use them. But to answer your question, as far as I have been told, it would take a heck of a lot of solar panels.