Toyota is working with Sharp to use solar panels to produce electricity for the Prius.

Not long ago, Tesla CEO Elon Musk suggested the company would re-introduce its “solar roof,” which could power part of the vehicle, such as the HVAC to keep the interior of the car at an ideal temperature.

It seems now that Toyota is investigating the possibility of answering the obvious question: If you can power the roof, why not the whole car?

Initially the answer was that the solar panel roof couldn’t generate enough power to do that, but now technology has advanced significantly. The Japanese automaker is tinkering with ultra-thin solar panels used for satellites to produce a solar-powered Prius.

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Ideally, the panels would eliminate the need for this special Prius to plug in and charge up. The program is part of a demonstration project funded by the Japanese government. Using panels designed by Sharp Corp., Toyota engineers fitted them to the hood, roof, rear window and spoiler in hopes of generating enough power to push the Prius down the road.

Toyota’s solar-powered Prius can travel 35 miles under ideal conditions, but if its cloudy or too hot, it will not perform as well.

The electricity from the panels travels directly to the drive battery, allowing the Prius to charge while driving or parked. The results have been encouraging. The car has been charged up to travel as far as 35 miles, which considering the after commute for a U.S. driver is less than 30 miles, this could be viable transportation for some motorists.

However, the performance of the panels 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, the company notes.

The Sharp-produced panels are extremely thin at just 0.03 millimeters. Additionally, because they’re so thin, they can be easily shaped to the curves of the Prius. However, 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.

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Even though the panels are super thin, the truck of the Prius is weighted down with an extra 180 pounds of batteries that are charged by the solar panels. This isn’t Toyota’s first go-round with this concept. In 2017, the company tested its Prius PHV model.

By enhancing the solar battery panel’s efficiency and expanding its onboard area, Toyota was able to achieve a rated power generation output approximately 4.8-times higher than the commercial model Prius PHV equipped with a solar charging system.

The solar panels used by Toyota and Sharp are just 1 centimeter thick.

In addition to substantially boosting its power generation output, the demo car employs a system that charges the driving battery while the vehicle is parked and also while it’s being driven, a development that is expected to lead to considerable improvements in electric-powered cruising range and fuel efficiency.

Toyota is conducting the trials under various driving conditions in Toyota City, Aichi Prefecture, Tokyo, and other areas. While the updated test version has produced impressive upgrades, Toyota officials are issuing some familiar refrains when it comes to the current viability of the concept.

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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”.