Anyone can decide at any time to install a solar power system or energy-efficient windows and appliances — or otherwise improve the energy efficiency of a home or business. But the evidence is undeniable: Far more people go green if they get a tax break in return. Back in 2006, when Congress passed a $2,000 credit for builders to construct homes that use 50 percent less energy than the prevailing standard, builders responded despite initial concerns that no one would be able to meet such strict standards.