As much as 4,600 MW of new fossil power plants may be needed to back up the state\u2019s 33 percent renewables mandate, according to the California Independent System Operator. In the most sanguine scenario, however, the grid operator says there might not be any need for traditional backup power. \u201cIf we want to be risk averse\u201d more power plants are necessary, said Mark Rothleder, CAISO director, market analysis and development. He was a witness Aug. 16 at the ongoing California Public Utilities Commission\u2019s long-term power purchase hearings. While state policy moves toward a fossil-lite electricity future, the intermittent nature of wind and solar may require concurrent new investments in building and operating back up power, according to the grid operator. \u201cIt\u2019s a good bet\u201d those resources will have to be fossil, said Rothleder. The assumptions involved in grappling with the amount of new fossil backup plants are in flux and involve considerable uncertainty. Just how much is needed to ensure the reliability of a greener grid depends on various factors, including the amount of reliable energy efficiency, demand-response that reduces peak load, and power storage--be it pumped hydro, batteries and\/or another emerging technology. The matter is also impacted by the location, type, and mix of renewable resources. The flow of power from wind energy, for example, has greater variations than solar powered projects. Solar thermal projects are easier to control than photovoltaic facilities. In a more sunny take on the state\u2019s renewables\u2019 requirement, if energy efficiency and demand-response excel, then no new back up power plants are to be required, according to CAISO data filed with the commission last month. Although some environmentalists are pushing for large solar and wind installations--and concurrent new transmission facilities to transport electricity to customer centers--they are siding with the little-or-no-fossil back up scenario, according to Carl Zichella, Natural Resources Defense Council director of Western transmission. \u201cIs that the scenario we want to bank on?\u201d queried Rothleder. The state has yet to establish a \u201cneeds\u201d determination for new power plants beyond the 33 percent renewable mandate. That requirement is supposed to be set late next year by the commission.