Throughout history, humans have relied on energy from the sun. In ancient times, Greeks used mirrors to collect solar energy. Centuries later, in 1839, French scientist Edmund Becquerel discovered that sunlight shining on specific materials could cause a spark of electricity and, under certain conditions, form an electric current.

Since then, photovoltaic (PV) technology—in which silicon cells convert sunlight into electricity—has developed into a viable energy source. More than one hundred thousand U.S. homes, mostly in rural areas, exclusively use solar power. Utility companies in California, New York, Texas, and other states now install PV systems on residential rooftops. In other nations, primarily in the Third World, many homes that cannot be connected to utility grids because of high costs could eventually be powered by solar cells. Indeed, organizations such as the World Bank are investing tens...