In the 1990s, Willett Kempton, a professor at the University of Delaware, proposed in a paper that electric vehicles could help pay for themselves by selling power back to the grid. When no one jumped on the idea, he decided to develop the technology himself.

Now, the pilot project he spearheaded at the university in conjunction with power-plant operator NRG Energy Inc. brings in roughly $110 a month per electric vehicle. The operation uses software to link a minimum of nine electric vehicles, mostly Mini Coopers, together...