Wouldn't load-balancing flywheels at each substation be a more quantifiable and easily implemented solution than hundreds of thousands of car-chargers with secure internet connections to a smart grid and additional circuitry to flow power back into the grid? It sounds risky to lay odds that there will be enough cars connected to chargers near where you need the extra power ... and I shudder to think of the effect produced by a "zero-day" exploit giving a hacker control of the grid managing the remote power requests!