How California Grid Operators Managed the Eclipse

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Another consideration leading up to last week’s Great American Eclipse, how would the partial or total sun blockage affect rooftop solar customers?

IEEE Spectrum
Contact: Peter Fairly

Western grid operators such as California’s have become adept at hitting the electricity supply curve balls that weather-dependent wind and solar power toss their way. Today’s eclipse was more like a 105-mph smoker from a predictable fastball pitcher. Grid managers knew exactly when the moon would transit across the sun’s path, blotting out gigawatts of solar power generation. And they could project that, at its worst, the shifts in solar generation would be no more challenging than those experienced already on partly cloudy days.

California faced the biggest challenge. It is not in the path of totality, but the 56- to 78-percent blockage of sunlight will have a big impact thanks to 18 gigawatts worth of solar panels deployed at utility-scale power plants and across rooftops—more than any other state. California’s Independent System Operator (CAISO), which manages most of the state’s power grid, projected this morning that the eclipse would knock out about 4,300 megawatts (MW) of utility-scale solar and another 1,300 MW of rooftop generation. The utility-scale solar loss, traced on the CAISO website after 9 am Pacific time until the eclipse’s California peak at 10:22 am, was actually 3,400 MW.

Executive Director of System Operations Nancy Traweek told reporters at CAISO’s Fulsom control center that the speed at which solar generation was expected to crash and rebound—up to 70 MW per minute—was something CAISO had contended with on certain days in the past when moving cloud cover overlapped with sunrise or sunset.

CAISO expected to manage the quick loss and return of solar power with its established sources of flexibility. Those include rapidly-adjustable hydroelectric and gas-fired power plants, a regional power market that is purpose-built to balance quick-shifting renewable energy, and a growing capability to store electricity and otherwise shape demand for power in real time.

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