Los Angeles, California [The Wall Street Journal] The numbers are starting to look promising. For years, the big criticism of alternative energy was cost: It was too expensive compared with energy based on traditional fuels like coal and natural gas. Even though the fuel was often free — such as wind or the sun’s rays — alternative-energy producers had to plow lots of money into finding the best way to capture that energy and convert it into electricity. Fossil-fuel producers, on the other hand, could draw on billions of dollars in infrastructure investments and decades of know-how. Now the equation is showing significant signs of change. Costs are falling for some alternative-energy sources, driven by new technology and renewed development interest. –Leila Abboud and Jeffrey Ball contributed to this article.