We analyze the effect of charging a significant number of plug-in hybrid vehicles (PHEVs) in the United States using presently available night-time spare electric capacity in the shortterm and new base-load capacity in the long term. Nationwide, there is currently ample spare night-time utility capacityto charge even a large fleet of PHEVs. Using the mix of generating plants expected to be used for PHEV charging, we find that, while driving on battery power, PHEVs compared to their conventional hybrid counterparts reduce CO2 emissions by 25% in the short term and as much as 50% in the long term. The shortterm fractional increase in demand for margin fuels such as natural gas is found to be roughly twice the fractional penetration of PHEVs into the nationwide light-duty vehicle fleet. We also compare, on an energy basis, the CO2 savings of replacing coal plants versus replacing conventional vehicles with PHEVs. The result is found to depend critically on the fuel economy of the vehicles displaced by the PHEVs.