I’m almost certain 20 miles range gained in 8 hours is not realistic. The website claims the average for this car is 112km (69.6 mi) per week. The claim of up to 245km per week is a bit misleading.

Looking at the chart they provide, that assumes perfect conditions in the month of June only. A sunny week in December will only provide 70km.

But going off the stated ‘average’ of 69.6 miles a week, roughly 10 miles for a whole day of collecting sun. You might get nice returns on a sunny day in the summer, but in general year round, your probably lucky to get 3-5 miles.

Elon Musk spoke about this on Joe Rogan recently, and said it’s just not worth it. The return on investment is just so small, and we’ve essentially reached the technical limits of what solar panels can provide. So we will likely never see this in a Tesla.