Toyota is secretly developing a car powered by solar cells mounted on the vehicle’s roof, according to a report today from Japan’s Nikkei newspaper.

The report said the automaker hopes the vehicle will eventually be totally powered by the solar cells—but that would take many years. Prior to that time, energy from the sun could provide only some of the power for the vehicle’s batteries and other electric accessories.

The report didn’t cite sources and Toyota executives were unavailable for comment. In July 2008, media organizations speculated that Toyota would offer a rooftop solar panel as an option on the next generation Prius. Energy experts believe there’s a mismatch between the amount of solar power that can be delivered and how that energy might actually be used by the current Prius. A Prius solar panel will likely be limited to running a small ventilation fan while the car is parked.

Viable Solar-Powered Precedents: Lacking

A number of niche automakers have unveiled solar-powered concept vehicles in recent years. To correct the mismatch between the energy that could be produced and what is required, carmakers try to expand the available space for solar cells while reducing the weight of the vehicles. If the unconfirmed Nikkei report proves correct, Toyota could be the first major automaker to pursue a solar-powered design.

Venturi Eclectic
Mindset Six50

Top: The Venturi Eclectic, shown at the 2006 Paris Motor Show, is charged by the 2.5 square meters of solar cells on its roof. A complete charge of the nickel metal hybrid batteries provides a range of about 30 miles.
Bottom: Mindset AG, a Swiss company, announced its plans in 2008 to produce an ultra-lightweight hybrid vehicle with roof-mounted solar panels. According to the company, the hybrid “Six50,” features a roof equipped with solar panels that power the lithium ion batteries, providing around 60 miles of driving.

The current Toyota Prius hybrid—with a 1.3 kilowatt-hour battery pack—has limited ability to store energy to power the wheels. A Toyota plug-in hybrid or electric vehicle with expanded batteries would be better suited to using solar-generated energy.

In fact, Toyota will debut a small battery-electric concept car at the 2009 North American International Auto show next week. “This is a pure electric vehicle,” said Toyota spokeswoman Jana Hartline. “It’s a concept we are bringing to the show basically to confirm our interest in electric vehicles.”

Toyota’s plans to harness power from the sun could get a boost when Panasonic Corp., Toyota’s partner in advanced battery production, completes its planned takeover of Sanyo Electric Co., a leader in solar technology. Toyota uses solar panels at its Tsutsumi plant in central Japan to produce some of its own electricity—roughly enough to power 500 homes.