I came across this paper a while back:
https://www.researchgate.net/publica...Solar_Vehicles
Since it was written the cost of solar panels has dropped by >90%!
We've seen eletric cars like Sono Sion and Lightyear One have solar panels, but it makes more sense to put solar panels on a non-plugin hybrid since energy in one of those is more valuable, about $0.12/kWh for electricity from the wall and $0.77/kWh for electricity in the car - more than six times as much! (with European fuel prices)
Recently the Hyundai Sonata and Kia Optima hybrids have come out with solar roof options.
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2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)