A project led by Toyota Motor is experimenting with a sun-powered Prius that it hopes will one day require no power plugging in.

In the Japanese government-funded demonstration project, Toyota engineers fitted solar panels designed by Sharp to the hood, roof, rear window and spoiler to see how much solar energy 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 56 kilometers of travel.

But the performance drops off quickly if it is cloudy or even when it’s too hot. If used in real-world driving in those conditions, the Prius would have to be plugged in to recharge.

The solar cells are just 0.03 millimeters thick, 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 an extra weight of around 80 kilograms (180 lb).

Satoshi Shizuka, Toyota’s lead engineer on the project, said that commercialization of the conceot likely remaines “years away”.