The issue I brought up was the fact that it would take a panel array of 4 feet by 8 feet above your electric Smart to only partly charge the battery. Worse yet, it would have to tip upward by your latitude AND face the sun in order to gather the maximum energy. That amount is not enough to fully charge the battery.
While a great deal of effort has been made in improving solar cell efficiency, currently 25% is about as good as it gets and most panels aren't that good; roughly 15%. An efficiency of 100% is considered impossible, but some research into this suggests that we might be able to go as high as 50%. Even with this high an efficiency, it's still not enough to fully charge the battery after one full day in the sun. The south western states get the most average sunlight in a day and the north as well as the eastern states fall short of this amount. (The area near Florida is the exception.)
If we hope to charge the car battery, a much larger set of panels on the roof of a house or office will be required. The problem is that the car must be plugged into these panels in order to charge the battery.
One thing I forgot to point out was the battery efficiency, which is roughly 70%. When I stated the following:
Now the Smart Electric battery capacity is 17.6 kWh, so a whole day of charging would charge the battery from 1/5 to 1/4 capacity. With a range of 76 miles, that's about 15 to 19 miles after a whole day of charging.
I forgot that it would take 25.1 kWh of energy to fully charge the battery. Thus the range would really be from around 10 to 13 miles from a full day's charge.
One could have a battery at the house that could charge during the day and be used to charge the Smart at night. The bad news is that each step loses 70% of the energy, resulting in 0.7 x 0.7 = 0.49, about half the energy is lost. This would require a 36kWh of energy to charge the battery using this approach.
Costs for installed solar have dropped; the price varies depending on the area, but it appears to be from $4 to $6 per installed watt. The solar panels are about $1 per watt, with the labor, permits, and mounting hardware taking up most of the instillation costs.