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Solar Panel Car Roofs: Are they a good idea?

Avatar for Ben Zientara
Published on 02/20/2020 in
Updated 02/20/2020
A Solar Panel Car Roof

Source: Lightyear Automotive

In Short: Solar panel car roofs are a relatively new phenomenon that promise to use renewable energy to add some charge to the car’s batteries, but short of some absolutely miraculous new physics breakthrough, a fully solar-powered car will not be practical for the vast majority of people. Still, some car manufacturers are pushing the envelope with integrated solar cells, and it’s worth taking note of what’s going on in the industry.

Have you ever seen a solar car? Not just a car that has solar cells in its roof, but a fully solar-powered car. It looks like this:

Agoria BluePoint Solar Car

Source: Agoria Solar Team

Not exactly the kind of vehicle you can count on to get your family to the grocery store and back on a Saturday afternoon, is it? The car you see above is the BluePoint solar car from the Agoria Solar Team, winner of the 2019 World Solar Challenge in Australia.

The BluePoint traveled 3,019 km (1,876 mi) across the continent of Australia powered only by the 2.6 square meters of solar cells embedded in its surface. But despite this feat, the power generated by those cells probably couldn’t get you to work and back in a day.

See, the BluePoint can hold a single occupant with no room for another passenger or any cargo. In fact, it’s tiny, with a total volume of something like 16 cubic feet (not counting its cutout lower section) and weighing less than 300 pounds. To put that in perspective, the vehicle most people in the US drive, a mid-sized SUV, takes up 36 times more space and weighs in 4,336 pounds.

Why aren’t solar powered cars practical?

A typical home needs a solar array covering 500 square feet to produce as much power as the people inside need in a year. Ideally, those panels are placed on a south-facing roof with an optimum tilt of about 30 degrees, give or take a few.

A few years ago, we did the math on how many solar panels you need to charge an electric car. Looking at the Nissan Leaf, at 50 miles per day, we came up with a total of 3.44 kilowatts of solar panels. Some of the best home solar panels on the market make around 345 watts under full sun. You’d need 10 of them to make enough solar energy to power the Nissan Leaf, a small sedan, on its daily journey from work to home, if you drive 25 miles each way.

That’s 250 square feet of solar panels on IDEAL roof space in order to make that much energy.

That’s why solar powered cars aren’t practical. There simply isn’t enough surface area on a car to allow the solar cells to generate enough electricity for the average driver, especially considering the surfaces of a car aren’t always pointed in the right direction or not under shade.

That impracticality isn’t stopping automakers, though. At least a few of them have added photovoltaic cells under special glass on their cars. Some manufacturers add a solar roof or hood, and at least one also puts cells into the door and quarter panels.

Here’s a bit more about solar car manufacturers:

Car Companies That Make Solar-Powered Cars

Two car companies either produce or will soon produce electric cars with solar panel roofs that charge the vehicle battery to run the electric motor: Sono Motors and Lightyear Automotive.

In addition to the two companies above, Karma, Hyundai, and Toyota offer hybrid electric vehicles with embedded solar cells that make some small amount of electricity. Again, these cars aren’t what we’d call “solar powered.” For example, the Karma Revero has 200W of solar cells in its roof, which would require full sun for 105 hours to charge the car’s 21 kWh battery and drive 50 miles.

Most places around the country get about 5-6 hours of full sun equivalent per day, which means you could drive about 2.6 miles per day using just solar energy from the Revero’s rooftop solar cells.

Sono Motors

An image of the Sono Sion Solar Car showing where the cells are located

Sono Motors is currently offering its Sion electric vehicle for pre-order for 25,500 Euro (about $27,500), if you live in Europe. There’s no telling when it will come to America. The car comes with a 35 kWh battery, capable of taking passengers an estimated 158 miles (255 km) per charge.

More importantly, the Sono Sion has 248 solar cells embedded in its roof, hood, trunk and sides. Simply put, it has more solar than any other car on the market. Each solar cell can generate an estimated 4.84 watts under full sun, and the whole surface of the car is rated to produce 1.2 kW.

There’s just one problem, with solar cells on the roof, hood, trunk and sides, only some of the solar cells will ever get full sun at any given time.

Can the Sono Sion charge itself with solar power?

Sono claims the Sion can get “up to 34 kilometers” (21.1 miles) of range per day using just the solar cells embedded in its surface. That’s a pretty sweet deal, but is it accurate?

Based on Sono’s estimate, the Sion gets 4.53 miles for every kilowatt-hour of energy stored in its battery. We ran the numbers based on the location of the cells in the Sion’s body through NREL’s PVWatts solar estimating tool, and found out (drumroll, please), that it’s possible that Sono is right.

Sono Sion Solar Cell Production Estimate of 5.32 kWh per day in Santa Fe, New Mexico

If you leave the Sion parked all day, facing northward, on a perfectly flat street in Sante Fe New Mexico (one of the sunniest places in the U.S.) with no shade, and don’t move it (deep breath), our estimates show an average daily charge of 5.32 kWh. Enough to go 24 miles.

But does anyone really use their car like that? You want to drive the thing, right? That means you’ll be driving in all different directions, and parking in places with shade, clouds, birds that poop on your car, and more. We’d say you’ll actually get mayyyyybe half that range from just solar on an average day in sunny places.

Lightyear Automotive

A rear view of the Lightyear One's Solar Panel Car Roof

The Lightyear One is a solar-roofed challenger to Sono Motors’ Sion, but the differences between the two are pretty stark.

For starters, the Lightyear One is currently available for reservation for 119,000 Euro (about $128,500), which is nearly 5 times the cost of the Sion. More importantly, the Lightyear One has a battery that can hold 60 kWh—less than twice that of the Sion—but because of the car’s sleek design, the One can get almost 3 times the range than Sono can offer.

The roof of the Lightyear One contains 5 square meters of solar panels (cells, actually), packed more densely than those on the Sion. Basically, it looks like Lightyear managed to fit about the same generating power as Sono did, but only on the roof and hood.

Solar cells on the Lightyear One's hood

Lightyear claims the One can get 12 km (7.46 mi) of range for every hour it spends in the bright sun, which would mean an average of about 30 miles per day in most places. And although we’re just as skeptical of that claim as we are of Sono’s, the increased efficiency of the Lightyear One makes it plausible that you could see that much production on a good day.

What about Tesla?

Tesla is notable for its cars’ lack of solar roofs, despite the fact that the company itself manufactures solar cells and panels for use on homes. The head of Tesla, Elon Musk, said in 2017 that a car is “the least efficient place to put solar,” and we tend to agree.

The Final Word on Solar Panel Car Roofs

Look, cars are just not ideal for solar panels. We need them to be mobile, go anywhere, and withstand not just outdoor weather, but wind whipping around at 60-80 mph, rocks kicked up from the road, bird poop, and whatever life throws at them.

We all look for shady spots to put our cars when we park them, so why add a reason to find the sunniest spot? Our cars end up spending lots of time in garages and covered parking ramps, which means a lot of time out of the sun.

You know about range anxiety? How about adding the anxiety of having to find a sunny spot just to eke out a mile or two on your commute home? How much further from your destination would you park to find that spot?

We love solar power, but we also understand the best place for it is fixed on a rooftop. Why not install solar on your home and charge your electric vehicle with solar power that is always generating every day, in the sunniest spot, right where it should be?

Last modified: February 20, 2020

4 thoughts on “Solar Panel Car Roofs: Are they a good idea?

  1. Avatar for J.J. Wolf J.J. Wolf says:

    I think charging at about 4 miles per hour you it starts to be significant. If you go to a remote location, but don’t actually need to drive it while you are there, then over several days you could get a significant amount of range.

  2. Avatar for gill gill says:

    The question should be: is it effective enough to be worth it. I imagine that on the lifespan of the vehicle solar panels could generate enough energy to counter balance production emissions and cost of installation.

  3. Avatar for M Mullen M Mullen says:

    Can any of these cars with supplemental solar actually use the energy they generate while they are driving or is it only useable while in Park? Probably the best way to use solar in a car is to run a small vent fan on hot days to help keep the cabin cool when parked in the sun. Because most parking lots don’t have much shade.

  4. Avatar for THT THT says:

    This line of argument is kind of similar to the argument of needing 400 miles of range in an EV. Yes, solar won’t completely power an EV’s energy needs, but why set that as the design point or goal?

    Every kilowatt-hour counts. If a vehicle has 1 kW capacity of solar on its roof, trunk and hood, that’s 2 to 6 kWHr of energy per day that could be put in a battery. That’s 2 to 6 kWHr per day not drawn from the grid. That’s 10%, 20%, 50%, even 100% improvement in a vehicle’s de facto efficiency depending on the daily commute and parking situation. Across the year, if an EV driver is charging at home, that could be 10% to 20% less energy used. That’s a win-win.

    You present the argument that people are better off putting the money into panels on the roof, but it isn’t an either-or decision. People can have both. Eventually every single sun lit service should be a PV surface of some kind. Residential batteries should be used to store energy. EVs should serve as battery backup for the house.

    Heck, when it is a available, I’d love it when I could put a passive radiative cooling system on the north side of my house to keep it cooler, and save even more on energy costs. It’s really all of the above. Solar everywhere, passive radiative cooling where solar not possible. It should be on cars, every roof, every building, every school.

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