Many studies have demonstrated the technical feasibility of meeting the majority1,2 or even all3,4 electricity demand with renewable energy resources. However, renewable energy generators, such as wind turbines and solar photovoltaics, introduce different grid management challenges than power generated with nuclear and fossil fuels. Currently, the grid currently must be sufficiently flexible to respond to unexpected fluctuations in energy demand. Using wind and solar based energy technologies, however, requires that the grid be flexible enough to respond to variability in energy supply because we cannot choose when they generate electricity. Some of this variability is predictable and some more uncertain, as described below.