Solar-Augmented EVs? Is this a Thing?

By Paul Gipe

Note: The following was part of an article debunking the idea of putting wind turbines on EVs to extend their range. My comments on possibly using solar PV as a range extender instead generated so much interest that I decided to give the topic its own entry. Some have opined that my estimates of potential solar generation are much too optimistic. Perhaps. But I wanted to get an order of magnitude estimate of the potential for comparison to our driving pattern before dismissing the concept out of hand as I had done with wind.

Putting solar cells on an EV to augment home or fast charging could become a thing, particularly in sunny places like California.

Solar cells can be incorporated into the skin of the vehicle. They neither increase frontal area nor the coefficient of drag and their performance is independent of speed. They work as well when the car is standing still as well as when it’s moving down the road.

Unfortunately, it takes a lot of energy to move a typical vehicle at highway speeds and that’s a lot more energy than solar cells themselves can produce. Even covering the surface of the car with high-efficiency cells is insufficient to power the car directly.

Solar cells can charge an EV’s batteries while the car is in motion. And therein lies an opportunity to slightly extend the range of the EV with solar.

More importantly, most passenger vehicles sit idle much of the day. It’s during this down time that the embedded solar can trickle charge the EV, gradually building up the charge. If the EV is used periodically for short trips to the grocers, for example, the solar cells could effectively charge the car sufficiently over time to power just such short commuter trips.

There’s a lot of fervent in the field as inventors tool up to build solar-augmented EVs. To make on-car solar charging as effective as possible various groups on both sides of the Atlantic are pushing to improve the EVs aerodynamics and lightening the vehicle to improve the overall efficiency.

Dutch startup Lightyear recently signed an agreement with supercar manufacturer Koenigsegg to build their expensive, but sleek EV. Lightyear claims the car can gain 70 km (40 mi) from the 5 m² of solar cells imbedded in the car’s skin per day based on summer insolation in the Netherlands.

Not to be outdone by the Dutch, German startup Sono Motors has embedded 7.5 m² of solar cells in the skin of its Sion. Some of the Sion’s solar cells are on the vertical panels of the doors as well as the roof and hood (or bonnet to the British). This gives the Sion 1.2 kW of solar capacity in what the company is calling Vehicle Integrated Photovoltaics (VIPV).

Sono Motors claim that the Sion will be able to charge the equivalent of 15-35 km (10-20 mi) per day under German conditions.

The Sion will be produced by contract manufacturer Valmet Automotive.

While Lightyear and Sono’s designs are otherwise conventional four-wheel vehicles, San Diego’s Aptera takes solar-powered EVs in an entirely different direction. Their teardrop shaped three-wheeler looks like an airplane without wings.

Aptera’s claims are also bolder than their competitors. The company claims its two-passenger EV won’t require regular charging during a typical daily commute because of the 0.7 kW in solar charging capacity.

With its front wheel fairings and bug-like shape, the Aptera looks like a tadpole driving down the highway with a claimed Cd = 0.13 or half that of the Tesla Model 3.

None of the solar claims by Lightyear, Sono, or Aptera have been independently verified. Despite that there is promise here.

Running the Numbers

Solar panels with optimal alignment typically can produce ~1,000 kWh/kW/yr at the best sites in Northern Europe or ~1,500 kWh/kW/yr in Central California.

Lightyear doesn’t report how many kW its 5 m² of solar cells represents, but if it’s proportional to Sono Motors’ Sion, it should be ~1 kW. One kilowatt of solar cells in an optimum alignment should be able to generate ~1,000 kWh/yr in Northern Europe and ~1,500 kWh/yr in Central California.

Solar cells in the skin of an auto will seldom be in optimum alignment. We don’t know how well they’ll perform in practice, but let’s discount their maximum production 50%.

So a car like the Lightyear could produce ~500 kWh/yr in Northern Europe and as much as ~750 kWh/yr in California.

How far will that get you? I can get about 4 miles per kWh in our Bolt. (Note that this is more than the EPA rating.) That would get the Lightyear 2,000 miles in Germany, say, or ~3,000 miles in California.

The Bolt is far from the most efficient EV, but it is relatively small. Even so, lightweight, aerodynamic vehicles should be able to do much better than the Bolt or even a Tesla Model 3.

In a recent Youtube video of the Aptera by Fully Charged (see Aptera – this Hyper-Efficient ‘Car’ Costs HALF as much as a Model 3!) the reviewer says the Lightyear can get 7 mi/kWh while the Aptera claims to achieve 10 mi/kWh.

None of the claims by Lightyear or Aptera have been confirmed by an independent testing laboratory. Let’s temper the claims somewhat and assume that these new solar cars can get 6 miles per kWh or an improvement of 50% on our Bolt.

If these assumptions are reasonable, the Lightyear would be able to drive ~3,000 miles per year on solar alone in Germany and as much as ~4,500 miles in California.

On average, you could drive the Lightyear 8 miles (13 km) per day in Northern Europe and 12 miles (20 km) per day in Central California on solar alone. Of course you could drive further in summer, but less in winter.

The gist? Solar in the skins of EVs makes sense. Will the solar cells on the EVs from these startups meet their projections? Probably not, but the solar electricity they can generate represents a not insignificant amount of the electricity needed to power an EV for an entire year. For our use case, solar embedded in the skin of an EV could meet from 30% to 40% of the 10,000 miles (15,000 km) we drive per year.

Now we just need some real world experience to show just how much electricity these vehicles can actually produce and how efficient they really are.