Despite their environmentally friendly design, there's always been one caveat to plug-in electric vehicles. While they don't emit any carbon dioxide as they're driven — in fact, all-electric cars don't have tailpipes — they do use electricity that's mostly generated by burning fossil fuels. These fossil fuels do, in fact, emit carbon dioxide, which means that anything using that electricity is indirectly causing pollution.
One clever way around this conundrum is to generate the electricity on the car itself, using photovoltaic solar panels. Solar radiation hits the Earth at an average equivalent to 4.2 kilowatt-hours of energy per square meter of Earth. Over the course of a year, this adds up to about a barrel of oil per square meter, which doesn't seem like much until you realize the Earth's surface covers about 510 million square kilometers, or about 197 million square miles.