Plug-in hybrid and electric vehicles could grow to be a greater source, or sink, of power than the entire American electricity generation system. Is that a good thing or a bad thing?

It's a question that utilities and policy makers have been grappling with as they prepare for what could be millions of new electric-powered vehicles to hit American streets. Unless those cars can be charged at times of lowest electricity demand, they could overwhelm the electric grid or require huge new investments in power generation.

But Jasna Tomic, new fuels program manager for Calstart, a nonprofit group promoting clean transportation, sees the conundrum as a big opportunity for such vehicles to serve as backup power for the grid, through so-called "vehicle-to-grid," or V2G, technology.

If a quarter of the nation's car fleet was to go plug-in hybrid or electric, the combined energy they could store would equal about 750 gigawatts, Tomic said Thursday at the Opportunities in Grid-Connected Mobility conference in San Francisco. Of course, that's a share of the market that could take decades to reach, but if it comes about, "That basically surpassed the size of the electric grid," she said.

And while the capital costs per kilowatt for that vehicle storage are likely to exceed the average rates that utilities charge for power, they're below those that utilities pay for peak power demand times, when they have to call on seldom-used backup sources to meet the peak, she noted.

So when thinking of the demand that electric vehicles will place on the grid, "There's no need to go only one way," she said. "We really could go both ways, and provide the power back to the grid. Think of the vehicles not only as transportation but as resources of power."

That's been an area of research for automakers, utilities and smart grid companies for some time.

A consortium including the University of Delaware, electric vehicle system maker AC Propulsion, utility Pepco, regional transmission organization PJM and demand response company Comverge is testing its own V2G technology (see A V2G Test: Pool Electric Cars for Grid Needs).

The Electric Power Research Institute is working with General Motors Corp. and Ford Motor Co. on projects (see Prepping for Plug-Ins to Hit the Grid).

And smart grid software developer Gridpoint, which in September bought vehicle-to-grid software developer V2Green, has tested V2G technology with Duke Energy and Xcel Energy's SmartGridCity project. (see Laying the Grid Groundwork for Plug-In Hybrids and Gridpoint Gets $120M, Buys V2Green).

Tomic said technologies like these could lead to new markets for electric vehicles that could help lower their cost, which is still seen as the main barrier to their widespread adoption.

Not that they could serve as baseload power, of course, since they'll need that to be charged up, she said.

But perhaps they could be linked up with utilities looking for new sources to cover their expensive peak power needs, she said. Several hundred thousand vehicles could likely provide enough power to serve utilities so-called "ancillary services" needs completely, she said.

And "You don't need a smart grid to do ancillary services," Tomic maintained. While utilities have focused their V2G plans on establishing two-way communications between utilities and homes or businesses via smart meters, V2G efforts could use cellular networks, radio signals or Internet connections to handle the task, she said.

Electric and plug-in hybrids could be used to store power from renewable energy sources like wind, which is mostly generated at night when demand is lowest, she said.

That's something that Danish utility Dong Energy, IBM and a host of other partners are working on right now in Denmark (see IBM Tests Smart Charging in Denmark).

But many hurdles remain between the state of electric vehicle infrastructure today and making those vehicles viable sources of backup power, Tomic acknowledged.

Not only do the vehicles in question need bi-directional power electronics, wireless communications and some form of on-board metering systems to handle the task, she said.

They will also require a hitherto-unknown level of cooperation between utilities and automakers, and those two have "never had anything to do with one another," she said.

Joel Pointon, manager of electric transportation and clean transportation for San Diego Gas & Electric, noted that automakers will be concerned about subjecting car batteries to more frequent discharges and recharges involved with being used as backup power, which will lower their life spans.

Michael Tinskey, manager of hybrid and battery electric vehicle applications for Ford Motor Co., agreed that right now Ford has no way to account for that in its warranty system.

"We need to learn a lot more before we jump into that arena," he said. "The utilities aren't ready, and neither are the car companies."