The administration used data from the California Independent System Operator, which manages the electricity grid across 80 percent of the state and part of Nevada. Between 11 a.m. and 2 p.m. on March 11, almost 40 percent of the electricity flowing across the ISO grid came from large-scale solar power plants, a record. […] homes and businesses in the area served by the ISO grid now have enough rooftop solar panels of their own to generate up to 5.4 gigawatts of electricity. Factor in the electricity they produced for their owners on March 11, and solar met half of the overall electricity demand in the middle of the day, the administration estimates. Add in electricity generated by wind farms, geothermal plants, biomass plants and small hydroelectric dams, and together, renewable sources briefly accounted for 56.7 percent of all power on the grid on March 23, said ISO spokesman Steven Greenlee. The surge in renewable power, while a key part of California’s fight against climate change, does create its own set of problems. Plants burning fossil fuels use that heat to generate steam, which then runs through a turbine to produce electricity. The flood of midday solar power has even caused wholesale electricity prices in California to periodically drop below $0 per megawatt-hour. In the long term, many analysts say California will need affordable large-scale energy storage technologies to meet its 50 percent renewable energy target.

Source: Solar hits big, brief milestone in California – SFGate