A year-long trial in Denmark is showing that utilities can use parked electric vehicles (EVs) as spare batteries, and those EVs can earn quite a bit of money for their owners from the utilities.

In an interview with Bloomberg New Energy Finance, Nissan Europe’s director of energy services, Francisco Carranza, said that a fleet of 10 Nissan e-NV200 vans has earned €1,300 ($1,530) over the year.

Electricity grids around the world are facing an era of rapid change as more electric vehicles hit the road and as grid supply changes. For grid managers, sometimes small amounts of power are necessary to regulate current frequency and keep the grid working. At the same time, if a lot of electric vehicles draw power from the grid concurrently (for example, when they’re parked at home at night, or when they’re parked at work during the day), that threatens to change how grid operators plan to meet demand, as well.

Researchers and grid managers have theorized that vehicle-to-grid connections could help solve some of these problems. By installing two-way connections where an EV could charge its battery and send power back to the grid when it’s needed, an electric car battery increases its value and makes electricity infrastructure more stable.

Research has been done on vehicle-to-grid connections for years. In 2010, East Coast grid manager PJM worked with the University of Delaware to test communication control and logic between an electric vehicle and a grid. More recently, Ford and GM have tried similar tests, working especially to synchronize charge and discharge cycles so that the owner of the EV always has enough juice to get in the car and go when necessary.

The latest Danish trial was started last August as a project between Japanese automaker Nissan and the Italian energy company Enel. Essentially, 10 Nissan e-NV200 vans were hooked to a two-way grid connection during the day on the lot of the Danish utility Frederiksberg Forsyning (much like the GM trial mentioned above, utility employees were the beneficiaries of the trial here). The two-way grid connection would allow the cars to draw electricity when their batteries are low and send electricity back to the grid if necessary.

The total capacity of the Nissan fleet when they’re all connected to chargers is 10kW. The platform that the chargers use to intelligently send power between the grid and the cars, incidentally, was provided by a California company called Nuvve, which commercialized technology that was developed by the University of Delaware.

Although vehicle-to-grid systems are in their infancy and real results won't be achieved until large numbers of EVs are connected to grids, trial runs need to start happening now. “If you [blindly] deploy in the market a massive number of electric cars without any visibility or control over the way they impact the electricity grid, you might create new problems,” Carranza told Bloomberg New Energy Finance.