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Sonomotors have an electric car, the Sion. They claim that this car use its own solar-cells to charge the batteries in one day sufficient to drive up to 34km.

Their information sheet says:

Thus, under ideal conditions, up to 34 kilometers of additional range per day can be achieved with pure solar energy - CO2 neutral and completely for free. [...] For solar integration, we use monocrystalline silicon cells that produce energy even under cloudy skies or in the shade. viSono generates up to 1.2 kilowatts at peak performance.

This comes with a graph showing the calculated additional range in day per kilometer, based on average meteorological data for Munich, Germany:

enter image description here

But note the disclaimer:

All details provided for Sono Motors’ products are preliminary. Information about the vehicle is based on the Sion prototype’s current planning and technology at the time of publication (of this document). [...]

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    I'm also following what's happening to this car, (driven one,) and contemplate buying it.They have more detailed information on their site explaining this number: it ranges from 4km in January to 28km in July (in Europe; and yes, it's power required to drive this distance). Since it's their own data, I wonder if that counts as an answer (in that case I'll write it). Please also note that this car is still in the prototype stage.
    – user22865
    Feb 13, 2019 at 9:47
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    @JanDoggen Yes, if you can back-up there claim with simple physics calculation taking into account current solar-cell efficiencies. Also, do you know for which velocity they calculated that estimated distance ?
    – james
    Feb 13, 2019 at 10:37
  • I'm editing their more detailed claim into the question, because I could not find independant information.
    – user22865
    Feb 13, 2019 at 11:15
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    Also, if I do the "simple physics calculations" that's probably not good enough as an authorative answer (the inhabitants of this site do not like back-of-envelope calculations).
    – user22865
    Feb 13, 2019 at 11:21
  • This seemed like an natural, inevitable innovation for electric vehicles. To the degree that proprietary, prototype claims can be verified, it would not be surprising, to me.
    – PoloHoleSet
    Feb 13, 2019 at 16:25

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Implausible with today's technology, but plausibly within reach of the near future.

It's hard to know, today, if this is true, but we can consider if it could be true.

How much energy is required? "Most electric vehicles cover between 80 and 100 kilometres with 10 kWh." So for 34 km we'll need at least 3.4 kiloWatt-hours, assuming this new car is in the same ballpark.

How much solar panel is required? "The average-sized solar panel takes up an area of 17.6 square feet and produces 265 watts under direct sunlight. That translates to just over 15 watts per square foot."

So we need 2266 square-foot-hours, which is kind of an awkward unit. If we assume that we can charge this up over 12 hours on sunlight, then we'll need about 190 square feet of solar panel -- a square 10 feet wide and 19 feet long.

That sounds a bit implausible to bolt on top of your electric car as is, but it might be plausible to assume that advances in aerodynamics, energy storage, solar panel technology, etc, might get us improvements of %200, which is starting to sound feasible.

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    Not sure if "average solar panel" should be the standard for a guesstimate. Something like this definitely would be something other than what people traditionally think of as a solar panel, I'd think...... news.wisc.edu/…
    – PoloHoleSet
    Mar 4, 2019 at 18:21
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    I don't think this site accepts "back of the envelope" answers like this one; if it did, you should use the actual spec of the car instead of generalisations from the web (e.g. 21% efficiency instead of "average" PV cell efficiency from some website, where the assumed average appears to be about 16.5% efficiency)
    – user56reinstatemonica8
    Mar 4, 2019 at 18:22
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    Your back-of-the-envelope calculation has a mix of unjustified and incorrect assumptions. For example, they're claiming the 34 km for Munich in June, which gives 16 hours of sunlight. They're claiming the use of mono-crystalline solar cells, which are more efficient than the average poly-crystalline cell. And their pictures show lots of square feet of solar panels. If they can manage an above-average energy efficiency, this falls solidly into the realm of "plausible" (at least, under marketing conditions. Your Mileage May Vary.)
    – Mark
    Mar 6, 2019 at 3:29

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