From cnet's Green Tech blog:
Someday, the electricity grid will operate with the equivalent of a giant hard drive. But in the short term, grid storage will look more like a PC's cache or RAM, able to serve up small bursts of power to keep things from crashing.
A panel of experts, organized by the New England Clean Energy Council, earlier this week said that the utility storage field has enormous potential. But rapid deployment of storage devices is held back by concerns over technology risk and financial complexity.
Technology optimists say that wide-scale energy storage will change the face of the transmission grid and make wind and solar power more compelling economically.
In this scenario, utilities store electricity made from renewable sources or produced during off-peak times. Then, when demand for electricity peaks in the middle of the day, they could draw from the stored-up charge....MORE
Here's their slideshow: