MIT Technology Review has a post on something that is pretty obvious to a data center crowd. Plugging in electric cars can create stress on a local circuit. In the data center users don't think about at the local power constraints on a circuit. Having an even distribution of power use on circuits is ideal.
The trouble arises when electric car owners install dedicated electric vehicle charging circuits. In most parts of California, charging an electric car at one of those is the equivalent of adding one house to the grid, which can be a significant additional burden, since a typical neighborhood circuit has only five to 10 houses. In San Francisco, where the weather is cool and air conditioning is rarely used, the peak demand of a house is much lower than in the hotter parts of California. As a result, the local grid is sized for a much smaller load. A house in San Francisco might only draw two kilowatts of power at times of peak demand, according to Pacific Gas & Electric. In comparison, a new electric vehicle on a dedicated circuit could draw 6.6 kilowatts—and up to 20 kilowatts in the case of an optional home fast charger for a Tesla Model S.
The most useful data I found to give you an idea of what is going on with Southern California Edison's ability to support car charging is the rate structure set up for electric vehicles. You would expect that the use of electric vehicles pushes most people to the Tier 2 rate. Charging off-peak vs. midnight -6a is three times more. Charging during 10a - 6p vs. midnight - 6a is 5 times more expensive during the summer.
I would say the Utility is driving the a more manageable electric load by creating the financial incentives. When people see their electricity bill it will get them to adapt their behaviors.
There is also a rate plan just for electric vehicles.