> 2.86 miles per day is the practical MAXIMUM, given perfect conditions
In your particular setup.
A typical car can expose about 3 square meters of lateral area for those same 8 hours, and receive 3 kW of irradiance. multijunction cells can exceed 50% efficiency, so we're talking about a theoretical upper limit of 12 kWh electric per day.
That would require a vehicle totally covered in cells, including the windows, so not very practical, but adding up to 30 miles/50 km per day is nothing to sneeze at.
We could also imagine all sorts of solar receivers that engage during parking and inflate the apparent surface within the limits available, track the sun etc. to maximize energy.
This is all wrong.
> multijunction cells can exceed 50% efficiency
The maximum demonstrated efficiency of a multijunction cell, in a lab, WITH CONCENTRATION is less than 50%. Commercially available cells are lower.
Concentration is an important caveat for two reasons:
First, it implies that you are collecting light from a larger area than the PV panel itself. Second, efficiency grows with increased irradiance (so efficiency will be lower without concentration).
> 3 square meters of lateral area
Lateral area is meaningless. It’s all about area perpendicular to the solar axis. Unless you are driving a box van or a big pickup truck, there is zero probability that you can put 3 kW of irradiance on your panels. Neither of those vehicles will achieve kWh/mile numbers anywhere close to a Prius.
In practice, you need to halve the efficiency and more than halve the collection area you quoted. You also need to account for conversion losses.