WASHINGTON — Finally, payback for the plug-in.

A line of Mini Coopers, each attached to the regional power grid by a thick cable plugged in where a gasoline filler pipe used to be, no longer just draws energy. The power now flows two ways between the cars and the electric grid, as the cars inject and suck power in tiny jolts, and get paid for it.

This nascent form of electric car commerce will be announced on Friday by the University of Delaware, the regional grid operator and an electric company. They have developed a system to collect payments for work (balancing supply and demand moment to moment) that is normally the domain of power plants.

The possibilities of using electric cars for other purposes are being realized around the globe. Electric cars like the Nissan Leaf and Chevrolet’s plug-in hybrid Volt, are generally not sold in the United States with two-way chargers that could feed back into the grid. But Nissan is offering a similar device in Japan that allows consumers to power their houses when the electric grid is down.

In the Delaware project, each car is equipped with some additional circuitry and a battery charger that operates in two directions. When the cars work with the grid, they earn about $5 a day, which comes to about $1,800 a year, according to Willett M. Kempton, a professor of electrical engineering and computing. He hopes that provides an incentive to make electric cars more attractive to consumers, and estimates that the added gadgetry would add about $400 to the cost of a car.