American Electric Power

A "megawatt-class" battery attached to American Electric Power's grid in Charleston, W.V. It's used as a buffer, able to generate a few hours of power for an overloaded substation.

These are sodium sulfur batteries from NGK Insulators of Japan. One of its battery purchases--part of a plan to buy 25 megawatts of storage this decade--will be used to store electricity generated by wind turbines next year.

These massive batteries don't come cheap. Installing three of them, capable of discharging 6 megawatts, will cost about $27 million. That's part of the reason that American Electric Power's use of storage on the grid is the exception, rather than the rule.