Fifty years ago, solar panels were so expensive that they were used mainly for powering billion-dollar space probes, and that's about it. But as the panels became cheaper — and as environmental worries about coal and other fossil fuels made renewable energy sources like solar more attractive — solar panels began sprouting up all over the place.
There are now enough solar panels installed in the U.S. to power about 11 million homes. Collectively, the panels produce about 1 percent of the country's total electricity. Experts expect that share to rise sharply over the next 20 years, as solar panels get cheaper and more efficient at converting sunlight into electricity.