The blog Camino Energy has a very detailed analysis of the payback on an electric vehicle (Nissan Leaf) compared to a conventional Toyota Camry. The author looks specifically at Northern California, where off-peak electricity prices are low enough that utilities could offer electric vehicle (EV) charging at 5 cents per kilowatt-hour (kWh). At that rate, with solely night-time charging of the EV and driving 12,000 miles a year, a Nissan Leaf pays back in 5 years.
The author provides a sensitivity analysis against higher electricity prices, and his entire post is worth reading.
