What Does ’100 Percent Renewable’ Really Mean?

In the race to build a greener electricity supply, some states are moving faster than others. The jockeying for position that accompanies this dash to become green, greener, greenest illustrates the dangers of making bold predictions in energy policy, which subsumes not only technical and economic questions, but political ones as well. This interplay of energy policy and politics escalated recently in an unusually heated public kerfuffle between scientists who study the evolving electric generation mix. A group of respected energy researchers released a paper in the journal Proceedings of the National Academy of Sciences that poked holes in a widely cited 2015 study of how to power the U.S. using only renewable energy sources. The new paper triggered a pointed response from the original study's author and triggered a lively debate in the energy blogosphere. In red states such as Texas and Iowa, market forces are driving rapid growth in wind generation. In blue states such as New York, policy is pushing the development of smaller, decentralized sources, most prominently rooftop solar. Another blue state, California, appears poised to adopt legislation that would require the state's utilities to obtain 100 percent of their electricity from renewable sources by 2045. A commitment to a 100 percent goal means that Californians will have to face difficult tradeoffs between reliability, affordability and environmental performance - tradeoffs that proponents of renewable energy frequently ignore and opponents often exaggerate. But cities such as Georgetown, Texas, and Aspen, Colorado, already claim to be meeting the 100 percent renewable standard. What, then, is so special about the proposed California law?