If you live in Tucson when 2018 rolls around, you could find yourself using one of the cleanest mass-transit systems in the world: the Solar Bullet. Engineers have proposed a 220-mph (354-kph) solar-power train to carry passengers the 116 miles (187 kilometers) from Tucson to Phoenix in half an hour [source: Alvarado].
It's still very much on the drawing board, but with more than 300 days of sunshine per year to work with, a solar-powered train is a brilliant if expensive proposal -- the first phase of the project would run about $27 billion [source: University of Arizona].
Those same 300 days could power a house for a lot fewer zeroes. Equatorial placement combined with few clouds, low humidity and a maximum monthly rainfall of 2 inches (5 centimeters) leads to a lot of solar-energy potential. Tucson's insolation value is 6.57, placing it at No. 4 on our list [source: CleanBeta].
No. 3 is the city at the other end of that solar train: Phoenix.
