Brad Plumer is relatively optimistic about solar's future. Tyler Cowen less so. Robin Hanson expects change to occur at a snails pace even as solar gets cheaper:
When solar is cheaper than coal or oil, that will include the cost of supporting infrastructure, such as building power plants. But since we’ll still have lots of old plants and infrastructure, we’ll still use a lot of carbon. And since places vary in the relative attractiveness of using carbon or solar power, we may even build more carbon plants after that point.
The Dish 