A typical “D” battery stores enough energy to deliver five watts of power for one hour. Now imagine more than five million such batteries strung together, and you get a sense of the storage capabilities of an advanced sodium-sulfur battery that PG&E plans to install later this year on its grid to support customer needs. It will be the largest battery storage system in California.

I guess that’s what they mean by the term “utility scale.”

The goal of PG&E’s battery storage project isn’t to operate five million flashlights or clock radios, but to provide backup power to customers in case of a power failure, improve power quality by smoothing out small variations in voltage and frequency, and help manage the ebb and flow of intermittent wind and solar power so the utility can handle more renewable energy.

PG&E’s planned battery installation, which just won funding support from the California Energy Commission, will have a projected life of 15 years. It will also support a 36-month demonstration project to study the value of storage in the utility’s distribution system.

“Energy storage will become critical as we migrate to California’s future ‘smart grid’ and integrate renewable energy sources, manage peak demand, and relieve transmission line congestion,” said James Boyd, vice chair of the Energy Commission. A 2008 report by the American Institute of Chemical Engineers said “massive energy storage . . . is a key to making the use of renewable energy possible on a broad scale.”

Besides the Energy Commission, PG&E’s partners in the project include the Electric Power Research Institute, which will help design and analyze results of the pilot project; NGK Insulators Ltd., which makes the batteries and promotes their use in a wide range of utility applications; and S&C Electric, which is handling design engineering and construction services.

Sodium-sulfur batteries run too hot to use at home or in your car. But they store a great deal of energy in a small space and have a long life, making them ideal for utility installations. In the United States, such batteries have been tested or used by American Electric Power, Long Island Power Authority and Xcel Energy.

PG&E is still working on the details, but plans to install the 4 megawatt battery at a site in Silicon Valley, where it will be easily accessible for study and where customers will appreciate its impact on service reliability. The goal is to have it operational by the fourth quarter of 2010.

Batteries are only one form of storage open to utilities. PG&E has long operated a pumped hydro facility, which generates power during the day by running water from a mountain reservoir through a turbine, then pumps the water back up into the reservoir at night when demand falls and power is cheap. PG&E is considering adding more such storage to its system.

PG&E also won funding last fall from the Department of Energy to pursue a project that will store energy during off-peak hours in the form of compressed air held in porous rock formations underground. As the air is released, it can be used to help spin turbines that will generate electricity. The project will be well-suited to storing excess wind energy generated at night.