One of the most important reasons for the rebirth of renewables took place at the state level, bearing out the famous adage of Supreme Court Justice Louis Brandeis that states can serve as the laboratories of democracy. Without a particular innovation introduced by individual staces—what are called renewable portfolio standards—it is doubtful that renewable energy in the United States would have seen the growth it has experienced since the new century began.

Daniel Yergin, The quest: Energy, security and the remaking of the modern world, New York, 2011, p. 587