Solar as major power source (Elon mentioned this quite a number of time) is something has to happen to support EV becoming majority of transportation. The current power grid won't be able to scale faster enough to support the EV growth.
We can do some math:
(1) According to NHTSA:
FARS Encyclopedia: about 3000 billion vehicle miles traveled per year, EV electricity cost is about 300wh per mile, which is 0.3Kwh / mile, 0.3Kwh / mile * 3000 billion miles =
900 billion Kwh.
(2) Total residential electricity consumption estimation of 2014 is
1407 billion Kwh, according to
How is electricity used in U.S. homes? - FAQ - U.S. Energy Information Administration (EIA)
900 / 1407=
64%, so that's 64% increase of load, it's very unlikely that the current grid can support a 64% increase in capacity. Not to mention, during the summer, when air conditioning is heavily used, it's also the time when people travel the most, etc, EV consumes more electricity during summer.
Then the logical thinking is grid/utility will have to increase the electricity price to reduce demand, which will encourage people to adopt the Solar+Battery (power wall) solution. And Solar+Battery (distributed grid) is probably the only solution, given the cost of upgrading the grid infrastructure will be super expensive and also time consuming.
I think this explains why it makes perfect sense for Tesla to acquire SolarCity,Tesla has to make Solar a major power source in order to achieve the sustainable transportation goal, so I don't think Elon will let the acquisition fail.