Toyota is experimenting with a sun-powered Prius, using new ultra-thin solar panels developed for satellites, 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 to the bonnet, roof, rear window and spoiler to see how much electricity the sun can generate, Reuters reported.

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 56km (35 miles) of travel, more than the 47km driven a day by the average American, according to a study by the AAA Foundation for Traffic Safety cited by the news agency.

But the performance drops off quickly if it is cloudy or 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 0.03mmthick, making them malleable enough to form fit the body of a car. 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 centimetre thick, Reuters noted.

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The boost is filled with batteries for the solar panels, adding around 80kg (180lb) of weight.

Making the entire package lighter and bringing down the extremely high costs are among the biggest challenges for the technology, Satoshi Shizuka, Toyota's lead engineer on the project, told the news agency, adding commercialisation likely remained "years away".

A solar cell roof is optional for the current Prius but it powers ancillary devices such as a ventilation fan to keep the cabin cooler while the car is parked.

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