WHEN Hurricane Frances ripped through Gainesville, Fla., in 2004, Christopher Swinney, an anesthesiologist, was without electricity for a week. A few weeks ago, Dr. Swinney lost power again, but this time he was ready.

He plugged his Toyota Prius into the backup uninterruptible power supply unit in his house and soon the refrigerator was humming and the lights were back on. “It was running everything in the house except the central air-conditioning,” Dr. Swinney said.

Without the Prius, the batteries in the U.P.S. unit would have run out of power in about an hour. The battery pack in the car kept the U.P.S. online and was itself recharged by the gasoline engine, which cycled on and off as needed. The U.P.S. has an inverter, which converts the direct current electricity from the batteries to household alternating current and regulates the voltage. As long as it has fuel, the Prius can produce at least three kilowatts of continuous power, which is adequate to maintain a home’s basic functions.

This form of vehicle-to-grid technology, often called V2G, has attracted hobbyists, university researchers and companies like Pacific Gas & Electric and Google. Although there is some skepticism among experts about the feasibility of V2G, the big players see a future in which fleets of hybrid cars, recharged at night when demand is lower, can relieve the grid and help avert serious blackouts.