Incredibly, today's grid has practically no storage capacity. The electricity coming out of your socket was generated less than a millisecond ago, so power plants have to continually generate enough energy for the biggest spikes. To prepare for the power fluctuations endemic to renewable energy, we'll need to inventory excess power to use during cloudy, windless afternoons and nights. The Massachusetts-based company Beacon Power's solution is to store the grid's surplus energy in hundreds of spinning carbon-fiber-and-fiberglass drums. Each of its Generation 4 flywheels features a 2,500-pound rotor mounted on magnetic bearings and sealed in a vacuum to create a near-friction-free environment. Energy coming in from the grid accelerates the three-foot rotor to 16,000 rpm (about Mach 2), where it keeps spinning with at least 97 percent efficiency. To pump energy back into the grid, some of the rotational energy is bled off to power a generator on the main shaft. Each flywheel can store a 15-minute, 100-kilowatt charge and can discharge 150,000 times over 20 years.