TOKYO — Inspired by new ultra-thin solar panels developed for satellites, a project led by Toyota is experimenting with a sun-powered Prius that it hopes will one day require no plugging in. In the Japanese government-funded demonstration project, Toyota engineers fitted solar panels designed by Sharp Corp to the hood, roof, rear window and spoiler to see how much juice the sun can generate. The electricity from the panels goes directly to the drive battery, so the Prius can charge while moving or when parked. On a good day, the charge can be sufficient for up to 35 miles of travel, more than the 29 miles driven a day by the average American, according to a study by the AAA Foundation for Traffic Safety. But the performance drops off quickly if it is cloudy or even when it's too hot out. If used in real-world driving in those conditions, the Prius would have to be plugged in to recharge. Toyota has experimented with solar panels on the Prius for years, but these new solar cells are super-slim — just 0.03 millimeters — making them malleable enough to form-fit to the body of a car. The engineers needed to create a buffer between the car and the cells to protect them, so the actual solar panel modules are closer to a centimeter thick. The trunk of the car is filled with batteries for the solar panels, adding extra weight of around 180 pounds. Making the entire package lighter and bringing down the extremely high costs are among the biggest challenges for the technology, said Satoshi Shizuka, Toyota's lead engineer on the project, adding that commercialization likely remained "years away".