"A company called Solar Electrical Vehicles is specializing in adding a convex solar roof to hybrid cars like the Toyota Prius, Highlander Hybrid and the Ford Escape Hybrid. The solar modules are rated at 200-300 watts, and this power is utilized to charge a supplemental battery. With the solar roof, the Toyota Prius can operate up to 20 miles per day in electric mode thus improving fuel economy by up to 29% (depending on driving habits and conditions). The system costs $2000-$4000 and the payback time is said to be 2-3 years."
Using Solar Roofs To Power Hybrids (TreeHugger)