As more homeowners generate their own electricity from solar panels, they still need power from a utility after the sun goes down.
Automakers now say they may have an answer, by storing that carbon-free energy in electric car batteries for later use.
Honda recently introduced an experimental house in Davis, Calif., to showcase technologies that allow the dwelling to generate more electricity than it consumes.
It is one example of the way solar companies and carmakers are converging on a common goal: to create the self-sufficient home, with a car's battery as the linchpin.
Car companies increasingly view all-electric and hydrogen fuel-cell cars as vehicles that will meet environmental mandates and lead to the development of new energy services and products beyond the garage.
Ford, Tesla and Toyota are pursuing similar strategies.
"It's a new world in terms of vehicles operating not as isolated artifacts but as being part of a larger energy system, and I think the greatest opportunity for automakers is figuring out how their vehicles become part of that system," says Daniel Sperling, director of the Institute of Transportation Studies at the University of California, Davis, which provided the building site and the heating and lighting technology for the Honda Smart Home.
The heart of Honda's 1,944-square-foot home is a room off the spotless garage that contains a 10-kilowatt lithium-ion battery pack housed in a black box. The battery is a smaller version of the one that powers the all-electric Honda Fit parked nearby.
Next to the battery pack sits a bigger white box called the Home Energy Management System. It is the brains of the house. It decides when to tap renewable electricity generated by a 9.5-kilowatt solar panel array installed on the home's roof to charge the car's battery or store the solar energy.
The rooftop solar array is about twice the size of one typically found on a comparable suburban home. The amount of electricity generated by the solar panels and stored in the battery pack allows the home to operate independent of the power grid, if necessary.
The home sends excess electricity to the grid. And if the utilities become overloaded, say, in the summer when temperatures spike and everyone turns on their air conditioners, the local electricity provider can send a signal directing the home to send solar electricity to the grid to help avert blackouts.
A similar size home would consume 13.3 megawatt-hours of electricity a year, while the smart home would generate an estimated surplus of 2.6 megawatt-hours annually, according to Honda.
"We can get our carbon footprint below zero," says Michael Koenig, the project leader for the Honda Smart Home, as he stood in the living room of the airy, light-filled house while a rerun of "McHale's Navy" played on a large flat-screen television embedded in a wall.
He held an iPad that wirelessly controlled all the home's functions, from lighting to the power systems, and that showed the house generating 4.2 kilowatts of electricity on a partly sunny morning while consuming 0.84 kilowatt.
"The system will calculate the household electricity load for the day based on the home's history as well as the expected solar output, and it'll only buy power at the lowest price," Koenig says.
The Honda Fit EV in the garage has been modified to accept energy directly from the solar array, too.
Steve Center, vice president for American Honda's Environmental Business Development Office, says Honda will focus on the potential to sell home energy management technology and battery systems to homeowners, builders and utilities.
"We see a lot of things converging," Center says. "There will be new business models, like home energy sharing and energy storage, using your car's batteries."