There’s a technology sitting idle in garages and driveways across America that provides a solution to its own potential problem. As more and more electric vehicles tap the grid, their giant batteries add to the system’s load. Timing is also a challenge: When people get home from work and plug their cars in, so too is everyone elsewhere switching on their own appliances, like washing machines and ovens and such.  But instead of being burdens to the electrical system, a clever trick is putting EVs on a trajectory to help save it . More models feature the ability to send their energy back to the grid in times of high demand — a trick known as vehicle-to-grid, or V2G — forming a vast network of backup power across a city. As demand wanes through the night, they charge up, ensuring an EV owner has enough juice to get to work in the morning.…