It sounds like a Utopian concept: drivers being able to power their vehicles simply by leaving them out in the sun. Even tech leaders know their bold visions have been skeptical. In 2017, Tesla CEO Elon Musk called it a fantasy to put solar panels on cars, stating that the surface area available for panels, and the amount of time many cars spend parked in garages, argued against pursuing the design.
"The least efficient place to put solar is on the car," Musk said at the 2017 National Governors Association meeting.
That has not stopped Toyota or Hyundai. Both automakers are debuting solar panels on their electric vehicles to add an additional charge, while auto start-ups are experimenting with even bolder solar panel designs.
The idea is not entirely new. The 2015 Toyota Prius had a solar panel roof feature that would help power the vehicles' ventilation system. Now Toyota is trying to take it a step further in their new solar power testing. The company is inspired by new ultrathin solar panels developed by satellites that make them malleable enough to form-fit to the body of a car.
"The goal is to utilize efficient power as much as possible," said Nathan Kokes, Toyota Motors North America Advanced Technology spokesman. "Solar offers a great opportunity to utilize the power of the sun to regenerate batteries."
In July, Toyota teamed up with the New Energy and Industrial Technology Development Organization (NEDO) and Sharp to begin public road trials in Japan for their EVs equipped with high-efficiency solar batteries. The vehicle features solar panels on its hood, roof, rear hatch door and the rear hatch garnish. The combined surface area covered in panels is 9 square feet.
The solar battery boasts an impressive 34% efficiency rating, compared to the average 15% to 20% efficiency rating most solar panels have (the efficiency is the amount of sunlight a solar panel can convert into electricity). Toyota hopes that added efficiency will help them reach their goal of adding 20–29 miles worth of solar energy to the car daily.
"When people drive, they're going to be outside," Kokes said. "So the beauty of this system is it will actually regenerate while driving. So as long as the sun is shining, you can generate electricity."
Range anxiety — driver fear of an electric car running out of power before a charge is possible — remains one of the barriers to broader EV adoption.
Toyota does not disagree with Musk on the basic science or limitations of solar power generation, but argues that any improvement in power is not to be dismissed.
"Toyota takes the approach that incremental change or incremental benefit is worth the effort," Kokes said. "It's not an all-or-nothing situation. Where I think Elon is coming from, potentially would be it won't charge enough of the car. But if you've got opportunities to make some impact, then it's worth the effort and continual study."
Kokes would not say whether the technology will be added to a planned production vehicle, or discuss pricing, but he did say Toyota would be evaluating the current test next spring.
In August, Hyundai released a version of its Sonata HEV with roof solar panels for the South Korean market.
"It is difficult for [the] solar roof to be popularized even if it develops further, because the solar energy delivered to the car is not much," Hyundai's team wrote to CNBC in an email. "However, in terms of auxiliary power, it was applied to the Sonata HEV because of its advantages ... which utilize the surplus area of the vehicle to generate power without pollution of fuel consumption."
Much like Toyota's prototype, the Sonata HEV will use solar energy alongside the power of a 12V (or higher voltage, depending on the model) battery. The solar energy can be used to both help with propulsion as well as power features in the vehicle, such as lights and air-conditioning.
The big selling point for solar panels on vehicles is the ability to make cars not just consumers of energy, but also producers. With six hours of solar panel-charging daily, it is estimated that up to 800 miles could be added to driver's travel distance a year.
But the benefits of solar panels on cars will depend on a car's physical situation, said Shannon Baker-Branstetter, manager of cars and energy policy at Consumer Reports. For example, it may be much less attractive for drivers who park in a garage than those who rely on street parking.
"Integrating it into the roof like that could have a chance at attracting some consumers," said Dan Edmunds, director of vehicle evaluation at Edmunds. He believes a car like the Hyundai Sonata HEV with roof solar panels could do well in the U.S. "People do all sorts of things for the feel-good factor, and [solar power] is certainly something that anybody can point to and anybody can feel good about," he said.
The company said the solar panels add approximately $1,075 to the cost of the car. North American car buyers can expect mass production of this Sonata to begin for their market in the first half of 2020, the company said.
It isn't just the big auto manufacturers experimenting with solar energy. In Munich, Sono Motors is working on a solar powered vehicle, the Sion.
Sono Motors was founded in 2016 by co-CEOs Laurin Hahn and Jona Christians, as well as creative director Navina Pernsteiner. Hahn and Christians started pursuing their dream of an electric vehicle powered by solar in 2012 after graduating from high school. Sion is the realization of that dream.
The car is covered in 248 solar panels, from the hood to the roof and even the doors. Sion's unique design was created to that no matter where a car is situated it is receiving the maximum amount of solar energy.
"It obviously depends on where you are living and where you are driving," said Mathieu Baudrit, head of research and development for solar integration at Sono Motors. "Even on cloudy days in Munich, you will get more than enough energy."
The company says the Sion will add up to an estimated 3,600 miles a year in driving distance from solar energy, with an average per day travel distance ranging from 3 miles to 14 miles, depending on the season.
Edmunds, however, is skeptical of the design, even if he believes people will be swayed by it.
"The three panels on the side doesn't seem like a very efficient arrangement," Edmunds said. "I can see what they are trying to do, but I have a hard time imagining that concept getting off the ground, not for many years."
Edmunds also feels that the Sion's solar design could be difficult and expensive to repair in the event of an accident.
Sion's solar system is embedded in a polymer, which Sono claims is highly durable and protects the solar cells from minor damage such as scratches. The company also says that the system's efficiency won't be compromised as long as a majority of the panels go undamaged in a minor accident, and that you would only have to replace a body panel in the event of a severe crash. Sono Motors declined to provide an estimate for how much a repair after an accident could cost. It is planning to make repair manuals and video instructionals for its cars widely available to make maintenance easier for owners and service providers.
Sono Motors says the car has already tallied over 10,000 pre-orders (a minimum deposit of roughly $550, at current euro to dollar exchange rates, is required) at its roughly $28,000 price point. Sono Motors expects production to begin in the second half of 2020.
Sono is focusing on European markets and has no current plans of introducing the car to the U.S.
Baudrit isn't worried about Sions collecting dust indoors.
"Most people in Europe don't have a garage or a house," Baudrit said. "The vehicle is meant to be shared. It should be parked in the street, not in the garage."
The broader business vision for Sion is a shareable experience — sharing rides, cars and the energy itself. Drivers will be able to use a goSono app to share their vehicle's energy with other Sion owners — essentially making Sion a moving battery. Drivers will be able to plug their Sion into another Sion and take energy from the other vehicle's battery.
"This isn't just about cost it's about the autonomy of the people." Baudrit said.
Musk has offered at least one bold idea — a "wrap of some sort" that could surround a parked car to make the most of a vehicle's surface area, in effect, a solar array that could be attached to a car. And Tesla, which owns its own solar manufacturing and installation business, is pushing the envelope on solar panel design in other structures, most notably the Tesla solar roof for residential homes.
Going green, and producing more renewable energy, may become a selling point for cars in the future, but Edmunds believes the cost might be too high.
"We're not talking about a lot of energy here," Edmunds said. "I'm not sure consumers would be looking to pay for that, unless there was some more obvious benefit. But it certainly is nicely integrated into the roof. It's similar to things we've seen before, although a little bit more expensive, but it seems like you're not going to get a lot out of it."
Edmunds also cited the EPA's current mindset of rolling back Obama administration fuel economy standards as another barrier for these cars to get around. The Trump administration is currently engaged in a legal battle with the state of California over the fuel efficiency standards. And the politics can always change.
"Certainly, in an environment where we're trying to reduce our CO2 output and improve fuel economy, you wouldn't ignore this," he said.
— By Jacob Douglas, special to CNBC.com