Fifteen electric Mini Coopers in the state of Delaware have just won official designation as a power plant.

How? When they are plugged in at charging stations, they can feed energy into the grid.

"We have a way of getting cars to talk to the people who manage the whole electric power system," said Willett Kempton, professor at the College of Earth, Ocean and Environment at the University of Delaware.

Kempton unveiled technology Friday that enables his fleet of limited edition electric cars to send and receive signals from regional grid operator PJM. The cars have two-way chargers that absorb energy or release it, depending on the signals.

"That allows us to use the batteries in the car to help to stabilize electricity," Kempton said.

Here's an example: What would happen when a major power plant fails?

"We would get a signal from PJM within a few seconds that would say 'We're short, discharge.'" Kempton said. "And so all the batteries in all the cars...would discharge and put more power on the electric system."

It can work in the reverse too, telling car batteries to charge when there is a surplus of energy in the grid.

This service is called 'frequency stabilization'. Right now, it is done by plants that burn fossil fuels. PJM Vice president for operations Mike Kormos said car batteries could take over some of that load.

"I think the vision is we could handle hundreds of thousands, if not a million cars, being able to provide this kind of service," Kormos said.

Kempton said the cars in his fleet earn about $5 a day when they work with a grid.

The regulation service they provide is likely to become more important as wind and solar energy, generally less steady and predictable in their generation patterns, ramp up.

Electric vehicles sold in the U.S. generally don't have two-way chargers. But Kempton has high hopes: He is in talks with BMW and four other companies working to outfit future electric cars with dual chargers.