The concept of the hybrid electric vehicles is nothing new; in 1903 the Krieger passenger car had both a gasoline engine and an electric motor. However, gas was cheap and the single engine gasoline-only cars have dominated the market for over 100 years. (Ref. 1)

A plug in Hybrid Electric Vehicle (PHEV) would be an upgraded model of the hybrids on the market today. They would feature an additional or expanded battery pack that may be capable of storing 10KWH of energy; this will provide them with an electric-only range of up to 30 miles (Ref. 1) while retaining the gasoline engine for extended trips.

source ( http://www.calcars.org/vehicles.html )

PHEVs could be recharged overnight from the existing electricity grid allowing them to achieve 70 -100 miles per gallon (ref. 3). If the electricity originated from a power plant that burns fossil fuels, the impact on carbon emissions may not be significant; however it would displace pollution from high traffic areas to the potentially more remote power plants. If nuclear power or renewable sources such as photo-voltaic or hydroelectric electricity was used to recharge the PHEVs the impact on carbon dioxide emissions could be significant.

Millions of PHEVS shifting transportation energy demand from gasoline to electric utilities will have a significant impact on the country’s electrical grid. In addition to conservation efforts, new power plants will have to be built.