The big number is 50. When companies can produce solar photovoltaic modules for less than 50 per watt, solar energy will be able to compete directly with coal. Right now, the cheapest solar cells are being produced for as little as 70 per watt. They are selling for about $1.26 per watt, with prices expected to drop to $1.17 next year. Most anticipate they they will hit 50 per watt within four or five years. As prices fall, demand is growing. Total solar installations in the second quarter [of 2011] grew by 69 percent over the same period in 2010. The number of Americans working in the solar industry more than doubled, to 100,000, from 2009 to 2011. That's considerably more than the 80,600 coal miners working in the U.S. Behind the price drops are cheaper manufacturing costs, lower costs for such crucial raw materials as silicon, and rapidly improving technology. Dozens of startups in the U.S. have potentially transformative ideas. The question is which can come out on top. The wide variety of companies developing competing technologies to capture and distribute solar power underscores the market's immaturity. Currently, researchers are experimenting with materials ranging from silicon to gallium arsenide to cadmium telluride, basing cost projections on disparate technologies that create solar cells. The goal is to build one that competes without government subsidies.