The smart grid is necessary infrastructure for the electric cars that will start to go mainstream in 2010, when GM plans to introduce its Chevy Volt for sale. Here's what GM says about the Volt's propulsion system: 

"[T]he first-generation Volt battery should hold 16 kilowatt hours of energy, have less than 300 cells, weigh 400-lbs, and take six or seven hours to recharge. As reported previously the Volt will purportedly be able to run approximately 40 miles on electric power alone."  Motor Trend, Aug. 15, 2008, available at: http://wot.motortrend.com/6285981/green/volt-watch-gm-gives-details-on-plug-in-hybrids-battery/index.html

(paraphrasing Denise Gray, GM's director of hybrid energy storage systems).