A growing number of plug-in hybrid electric cars and trucks could require major new power generation resources or none at all — depending on when people recharge their automobiles. A recent study examined how an expected increase in ownership of hybrid electric cars and trucks will affect the power grid depending on what time of day or night the vehicles are charged. In the worst-case scenario —- if all hybrid owners charged their vehicles at 5 p.m. — up to 160 large power plants would be needed nationwide to supply the extra electricity.