Worldwide, advanced technologies to store electrical power are on a sharp growth curve, say officials with the International Energy Agency (IEA). The growth in storage capacity, they note, mirrors expansion of renewable energy—specifically wind and solar—and is needed to back up the variable generation from these sources. A recent report takes a detailed look at U.S. storage and finds that in 2015 capacity grew by 243% compared with 2014. The analysis by GTM Research and the Energy Storage Association shows that in the last quarter of 2015 alone, advanced electrical energy storage capacity exceeded all similar storage installed in the U.S. in 2014 and 2013 combined. The 221 megawatts of storage capacity added last year doubled the previous capacity. Nearly all the additions were lithium batteries. The report predicts that U.S. storage capacity will grow 10-fold by 2020 and be a $2.5 billion annual market. IEA officials are reluctant to predict global storage capacity growth but stress that by 2020 global renewable electricity additions are expected to top 700 gigawatts—more than twice Japan’s current installed power capacity.