Cutting costs has been the key to solar’s rapid expansion this decade. The lion’s share of cost reductions in the solar industry has come from reductions in module prices. The U.S. $4 per watt you’d have paid in 2006 for modules alone gets you the entire residential solar system installed today, writes Andrew Savage, chief strategy officer of AllEarth Renewables and board member of the Solar Energy Industries Association (SEIA).