Car sizes vary a lot, but a full-size car in the U.S. is about 18 feet long and 6 feet wide, so it has about 100 to 110 square feet (9 to 10 square meters) of horizontal surface.

Why aren’t there solar-powered cars? –Emma, age 16, Springville, Utah. Solar cars exist. The best place to see them is the World Solar Challenge, a race that’s held every two years in ...