True solar-powered cars are actually electric vehicles that are powered by solar panels. The panels are used produce electricity by converting the sun's rays into energy, which is then stored in solar batteries. The car runs by using the energy that is stored in the batteries.
Currently, the solar cars available today still have several problems, as solar-powered cars are still a fairly new concept and research is continuing.
First, solar-powered cars have to be extremely lightweight and streamlined to be effectively powered by the sun. The problem is that this becomes a safety issue: ideally, cars would be sturdy and able to hold up safely in the event of an accident -- and lightweight solar cars aren't quite there yet.
Second, there's no such thing as a viable 100% solar car (yet!). The efficiency of solar cells makes it difficult to find enough space to install all the panels needed to power the car. And you can imagine, with the number of panels required, the cost just isn't viable and thus solar cars have not yet made it into the commercial mainstream. The average family simply wouldn't be able to afford to buy a solar-powered car, even if it were made available. In 2016, one website reported that a 'world-first' solar car under development could cost the owner $250,000!
Right now solar cars tend to have room for only one person (in a few cases, two people). This also makes it impractical for regular household use.
The closest thing you can find to a "green" car today is a hybrid vehicle. Although also fairly new, hybrid vehicles are becoming the new wave of the future in the auto industry. One company was reportedly considering placing solar panels on the roofs of hybrid vehicles. Perhaps this will start a new wave of solar-powered hybrid vehicles and one day lead to solar cars readily available to consumers.