For GSS Energy Use chapter 5. Excerpt:  ...Solar power is growing so fast in California — with installations by customers increasing tenfold since 2006 — that it is turning the state’s power system upside down.  In a twist that is being closely watched by power companies around the country, California utilities will install massive banks of batteries and other devices to store the power surplus created by solar panels in the afternoon, when the sun’s rays are strong. The batteries are then to begin discharging power into California’s electric grid in the early evening, around sunset, when the solar generation of energy dies down but demand rises as millions of people get home and turn on air-conditioners, televisions and other electricity gobblers.  The new system is the opposite of an idea utilities have considered for years: Use batteries to store power at night from traditional sources, like natural gas and coal, and run them down in the peak heat of late afternoon. ...At the end of October, the [California Public Utilities Commission] ordered the utility companies it regulates to install some form of energy storage equipment — exactly what was not specified — in the first mandate of its kind in the country. A critical purpose of the storage is to allow generators, which in California run largely on natural gas, to keep operating in the late afternoon, when the output from solar panels eliminates the need for their electricity.... http://www.nytimes.com/2013/12/24/business/energy-environment/catching-some-rays-in-california-and-storing-them.html. Matthew L. Wald, The New York Times.