In a February interview at TED2013,
First of all, he said, burning fossil fuels in a plant to generate electricity is more efficient that burning them in a combustion engine in a car. Because it is large and stationary, the plant can use the fuel much more efficiently:
In a stationary power plant, you can afford to have something that weighs a lot more, is voluminous, and you can take the waste heat and run a steam turbine and generate a secondary power source...
Even using the same source fuel, you're at least twice as better off.
Part two of Musk's argument is not based on numbers, but on the fact that electricity can be produced from renewable sources, most notably
We have to have sustainable means of power generation anyway, electricity generation. So given that we have to solve sustainable electricity generation, then it makes sense for us to have electric
Musk is also involved in SolarCity, which provides electricity generated by solar panels.
Here's the full talk, in which Musk also discusses his reusable rocket venture, SpaceX: