FIGURE 3. Ragone diagram of the discharge time at rated power (a factor in energy storage) vs. system power rating for a number of different electricity storage technologies; it is similar to many others that can be found on the Web. This one, ascribed to Nobelist Steven Chu and available at http://energy.wesrch.com/wiki-511-energy-storage-is-critical-to-grid-operations, has been augmented to include: (i) gravity storage; and (ii) stable synthetic chemicals (e.g., hydrogen gas, methane, or ammonia) that can be manipulated to produce mechanical energy. The maximum rates of charging and discharging of a storage module need not necessarily be the same.