One of the major benefits of plug-in vehicles is the ability to operate without any direct emissions while also diversifying the possible energy sources used for transportation. Unfortunately, that also means that unless you have your own solar or wind power installation, chances are you won't know where your power is coming from.
Unlike solar, wind and hydro power – all renewable and emissions-free – half of all electricity in the United States is generated by burning coal, with the next biggest share going to natural gas. With the fuel mix varying regionally, the net well-to-wheel emissions effect of plug-in vehicles will also differ. In some areas, overall emissions will go down significantly, in others they may actually get worse.
Researchers Michael Wang and Amgad Elgowainy from the Argonne National Lab have expanded the Greenhouse Gases, Regulated Emissions and Energy Use in Transportation (GREET) model to help the evaluate the overall environmental impact of deploying large numbers of plug-in vehicles. In addition to the fuel mix, the model will now be used to examine what happens if plug-in drivers charge their vehicles during the day instead of at night, as most advocates have assumed they will. Night-time charging could be largely accommodated with existing capacity, but extensive plug-in vehicle daytime charging might skew the mix toward even higher emissions. The variables are extensive and the only thing we know for sure at this point about the overall effect of plug-in vehicles is that we don't know all that much.