Toyota's third-generation Prius

Toyota's third-generation Prius, due at dealerships this spring, will have an optional solar panel on its roof. The panel will power a ventilation system that can cool the car without help from the engine, Toyota says.

But it's a long way from the 2010 Prius to a solar-powered car, experts told CNN. Most agree that there just isn't enough space on a production car to get full power from solar panels.

"Being able to power a car entirely with solar is a pretty far-reaching goal," said Tony Markel, a senior engineer at the federal government's National Renewable Energy Lab in Golden, Colorado.

In the new Prius, the solar panel will provide energy for a ventilation fan that will help cool the parked car on sunny, hot days. The question is, how much gasoline can solar power offset? According to recent articles in Japan's Nikkei newspaper, Toyota has bigger plans for harnessing power from the sun. Nikkei reports that Toyota hopes to develop a vehicle powered entirely by solar panels. Motorists don't have to wait for a 2010 Prius to drive a solar-enhanced car, however. Greg Johanson, president of Solar Electric Vehicles in Westlake Village, California, said his company makes a roof-mounted panel for a standard Prius that enables the car to travel up to 15 additional miles a day.

The system delivers about 165 watts of power per hour to an added battery, which helps powers the electric motor, Johanson said.

"Solar panels really need a lot of area," he said.

Leonhardt thinks Toyota's new Prius is a good first step toward using renewable energy. Johanson of Solar Electric Vehicles said he'd like to see Toyota bring the weight of a Prius down from 3,000 pounds to 2,000. He also hopes for a small gasoline engine and a larger electric motor. In the meantime, Solar Electric Vehicles sells its version of a plug-in Prius, with a solar panel installed, for $25,000, Bautista said.

Toyota is the largest automaker to incorporate solar power into a mass-produced car. But its solar panel is not the first for a car company. The Eclectic model costs $30,000, looks like a souped-up golf cart and uses roof-mounted solar panels to help power an electric engine.