Excerpt from New York Times article:

Power to the People: Run Your House on a Prius

Published: September 2, 2007
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.

...(Discussion and quotes from industry sources skipped)...

No automaker is selling a plug-in hybrid vehicle, but some ambitious people are making their own. Converting a stock Prius to back up the grid is much easier, and the guru for such conversions is Richard Factor, 61, an inventor from Kinnelon, N.J.

Mr. Factor says that small U.P.S. units, often used to provide backup power for computer servers, are inexpensive. His system, which he estimates would cost $2,000 to $4,000 to duplicate, incorporates a large U.P.S. mounted in his home and a long electrical cord to the Prius, where it connects through the car’s built-in relay terminals. His system is designed to integrate with the grid, but he said more rudimentary systems could be built for as little as $200.

During a recent six-hour power failure, Mr. Factor estimated that his 2005 Prius used less than one gallon of gasoline.

...(Honda naysayer unceremoniously ejected)...

You can (and should) read the whole article for non-PriUPS issues.  You may need to register with the Times to access it, or if you just search for my name and that of the author, you'll probably find a number of copies.