The time of day when plug-in electric cars are charged will determine the impact they will have on the national power grid, says a new study by Oak Ridge National Laboratory (ORNL). The Department of Energy lab study assumes that by 2025, one-quarter of U.S. cars will run on a combination of electric and liquid fuels and will require plug-in charging. If all cars are charged at 5 PM, when electricity demand is high, some 160 new power plants will be needed nationwide to supply the extra electricity, according to the study. However, it says, if the charging is done after 10 PM, when demand is minimal, as few as eight-or possibly no-new power generation facilities will be needed. Earlier studies, including one by Pacific Northwest National Laboratory, found that off-peak, idle capacity at existing electric utilities could provide enough power to fuel 84% of the U.S.'s 220 million cars if all of them were hybrid plug-in electric vehicles (C&EN, Dec. 18, 2006, page 40). ORNL's Stan Hadley, who led the study, says: "Utilities will need to create incentives to encourage people to wait. There are also technologies such as 'smart chargers' that know the price of power, the demands on the system, and the time when the car will be needed next to optimize charging for both the owner and the utility."