Although solar powered cars have been built, they are clearly not common. Why is this? Given that sunlight is a free source of power, why have solar powered cars not taken over the garages and highways of the world? The answer lies principally with power.
The rate at which an engine can provide energy is the power output of that engine. Usually automotive engine powers are measured in horsepower. A bog standard car engine will generally be able to generate 200 to 300 horsepower. The power output is a function of engine RPM for internal combustion engines and thus is not a constant specification of the engine. Now let us compare the power output available from solar. The Sun is of course putting out effectively infinite power, but we can only use the power that comes from light we can catch. Solar panels thus give a power output per area. Sunlight hitting a surface straight on delivers about 1000 Watts of power per square meter. Typical solar panels are about 20% efficient. So a solar panel of one square meter in area directly facing the Sun will generate about 200 Watts of power.
One horsepower is a little less than 750 Watts. Thus a typical one square meter solar panel in the optimum orientation relative to the Sun will generate about one quarter of a horsepower. A typical car will expose the most surface area to the Sun when the Sun is directly overhead, shining straight down on the roof of the car. Seen from above, a big car might be 18 feet long and 8 or so feet wide. That gives a surface of only about 13 or 14 square meters. Covering the entire surface of the car with solar panels would then generate about 3 horsepower.
Those 3 horsepower are when there are no clouds, the car is not in shadow and it is noon with the Sun directly overhead. You can see that there is no way solar power can deliver anywhere near the two or three hundred horsepower a gasoline powered internal combustion engine can. Solar power has a place in the future arsenal of silver BB energy solutions. But we are not going to see the cars of today run straight off of solar power!