TOKYO (BLOOMBERG) - Put together the best solar panels money can buy, super-efficient batteries and decades of car-making know-how and, theoretically, a vehicle might run forever.

That's the audacious motivation behind a project by Toyota Motor Corp, Sharp Corp and New Energy and Industrial Technology Development Organisation of Japan, or Nedo, to test a Prius that could revolutionise transportation.

"The solar car's advantage is that - while it can't drive for a long range - it's really independent of charging facilities," said Mr Koji Makino, a project manager at Toyota.

Even if fully electric cars overtake petroleum-powered vehicles in sales, they still need to be plugged in, which means building a network of charging stations across the globe.

The sun, on the other hand, shines everywhere for free, and when that energy is paired with enough battery capacity to propel automobiles at night, solar-powered cars could leapfrog all the new-energy technologies being developed, from plug-in hybrids to hydrogen fuel-cell vehicles, in one fell swoop.

But the current forecast is only partly sunny because there's still some work left to reach that level of efficiency.

"This is not a technology we are going to see widely used in the next decades," said Mr Takeshi Miyao, an auto analyst at consultancy Carnorama. "It's going to take a long time."

But it's not for lack of trying. Toyota and Hyundai Motor Co already introduced commercial models with solar panels on the roof, but they were too underpowered and could barely juice the sound system.

A Prius plug-in hybrid that sells for more than three million yen (S$38,000) offers solar panels as an option, but they charge the battery only when parked. The maximum amount of power for driving lasts only about 6km, said Mr Mitsuhiro Yamazaki, director at the solar energy systems division of Nedo.

Toyota has been testing a new solar-powered Prius since July, though it acknowledges that cars running non-stop without connecting to a hose or plug are still far away. Even so, the Toyota City-based company said the research will pay off in other ways.

Indeed, there have been some breakthroughs, mainly due to advancements by Sharp. The prototype's solar panel converts sunlight at an efficiency level of more than 34 per cent, compared with about 20 per cent for current panels on the market.

Because the solar cell being used by Toyota, Sharp and Nedo is only about 0.03mm thick, it can be placed on more surfaces, including the curvy parts of the roof, bonnet and boot. The electrical system can charge the vehicle even when it's on the move.

If the car is driven four days a week for a maximum of 50km a day, there's no need to plug into an outlet, Nedo's Yamazaki said.

Toyota's bet on solar is one of many that Japan's No. 1 carmaker is placing on the future of transportation. With Volkswagen AG and other vehicle titans facing uncertain futures as new technologies and business models ripple through the US$2.23 trillion (S$3 trillion) global industry, they're investing billions of dollars in cars powered by electricity and hydrogen to keep apace.

China has also explored efforts to tap the sun for transportation purposes. China tested an "intelligent highway" that buries solar panels, mapping sensors and electric-battery rechargers under transparent concrete, with the photovoltaic cells generating enough electricity to power highway lights and 800 homes.

Hanergy Holding Group introduced four cars powered by solar panels in 2016 as a way to expand the use of photovoltaic technology. The cars could be charged in daylight while driving, but they also contained lithium batteries for plugging in.

Nedo started the current project in 2016 with the goal of achieving an output of 1 kilowatt in vehicles by using a solar-battery module with a conversion efficiency of more than 30 per cent. Sharp, one of Japan's largest makers of solar panels, is developing the solar-cell technologies with Nedo funding, while Toyota is responsible for making the system to convert energy into driving power.

While the car's greatest potential markets will be sunny climes, such as California and western China, Nedo will evaluate whether it's also suitable for cities such as Tokyo, a key assessment for deciding whether to mass produce the vehicle, Mr Makino said. The current trial is scheduled to run through March.

If the solar-powered Prius can be made, it could compete with established manufacturers such as Hyundai and start-ups Karma Automotive and Lightyear. The Lightyear One is a US$170,000 luxury electric car that, according to its developers, can drive almost 800km on a single charge.

The start-up, founded in 2016 by a group of Dutch engineering students, uses 5 sq m of solar panels on the roof and bonnet that add more than 11km each hour to the car's driving range. Production is planned to start in 2021, and more than 100 cars have been pre-ordered, the company said.

Sharp's solar cells are more commonly used for charging homes and satellites. Toyota's demo car is covered by about 1,100 cells, each about the size of a business card, said Mr Hiroyuki Juso, a senior manager at Sharp's solar panel project. The expense of the cells makes mass production unlikely for now, he said.

"There is potential in this technology," Carnorama's Miyao said.