Electric cars and PHEVs may save the grid, rather than cause damage to it, by serving as power levelling reservoirs during peak demand and buffers during overproduction (ie at night).

Most American homes could run an electric car effectively for free, simply by reducing their household electricity consumption by about 30%. This would have very little impact on the grid.

For example, a typical (Prius sized) vehicle manages about 5 miles per kWh. Over 15,000 miles per year, a household would need to find 3,000 kWh to charge the car, or about 8 kWh per day.

8 kWh per day is easily saved by converting to low energy lights, AC systems, appliances and effectively insulating.

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm