This requires a two part answer.
Part one is how much energy can you get from solar cells placed on a car. The second part involves how much energy per km a road registered EV uses.
Solar energy collection by a car:
At the equator, at midday on March 21 or September 21 (that is, with the sun directly overhead) the spot you are standing on receives 1000 watts of solar energy per square metre.
This gradually reduces as the sun’s rays fall at increasing angles to the Earth’s surface. (i.e. morning, evening, increasing distances from the equator and time of year).
In Melbourne, this equates to 4.75kWh per square metre per day when averaged over one year.
If we are lucky, we should manage to cram two square metres of solar cells onto the horizontal surfaces of a typical small sedan.
Given that current solar cell technology converts approximately 14% of the sun’s energy to electricity, if this car is driven in Melbourne where it receives 4.75kWh on average per day on its solar cells, and converts this to electricity at 14% efficiency we will collect:
4.75 x 2 x 0.14 = 1.33kWh of electrical energy per day.
Energy use by EVs: Table two shows some typical Wh/km for production EV’s.
To sum up – if you park and drive the EV in full sun all day, andtravel less than 6 to 8 km per day (a bit more in summer, much less on cloudy winter days) then you may never need to charge your EV from the grid.
However, if you travel that little you could probably do without a car altogether and walk everywhere!