At the Future in Review Conference, Musk said that in 30 years, solar thermal and solar photovoltaic power will, combined, produce more electricity than any other source. That title is currently held (and held firmly) by coal. Displacing the coal industry with renewables would require massive capital investments and innovations, particularly in power storage.
I have to say, my mind doesn't have to stretch too far to see how it would be possible. But a few things need to happen first. Solar needs to get cheaper, and photovoltaics have to stop relying on raw materials (indium / monosilicon) that are difficult to acquire. And then we need to figure out how to store the power so we can use it at night. This could be through a combination of utility-scale power storage and distributed power storage through home fuel-cell and hydrogen creation systems.
Musk, as the chairman of Solar City, a company that installs panels on houses, sometimes with no down payment at all, obviously believe in the distributed power model. The goal of Solar City is to have people pay, not for the $30,000 panels on their roof, but for the 30 years of electricity those panels will generate. Already Solar City is projecting $80 M in revenue for this year.
The final piece of the puzzle in getting to solar supremacy came out in Musk's speech as well. Very simply, "There should be a carbon tax."
It's unclear that, without one, whether solar will ever be able to do any more than nip at the heels of big daddy coal.