Grid planners will be in tune when sun is eclipsed by the moon

The total solar eclipse which will blaze its way—in obscured fashion, of course—across the United States on Monday, August 21 is an almost once in a lifetime event. And it will impact generations both of the human and energy varieties.

The eclipse’s effect on U.S. solar energy output, however, will be unprecedented. Solar power on the grid was negligible when the last eclipse happened domestically in 1979, but now the combination of rooftop panels and utility-scale photovoltaic generates close to 40,000 MW nationally.

So what’s the grid to do when, as Pink Floyd crooned long ago, “Everything under the sun is in tune, but the sun is eclipsed by the moon?” The blot-out will have an important scientific outcome beyond its brief journey on August 21.

“When the sun goes away, so does the energy that powers our renewable solar panels,” California Public Utilities Commission President Michael  Picker, whose state generates about 18,000 MW from solar, said in a plea to get customers focused on conservation efforts that day.  San Franciso, for example, will be under the 75-percent portion of the eclipse radius

Kansas City Power & Light does not have California’s intensity of solar power—with 10 MW of utility-owned or leased capacity and an estimated 50-60 MW of net-metered solar from customers—but it’s centered right in the 100-percent zone of the eclipse as it moves diagonally northwest to southeast across the U.S. 

KCP&L’s sustainability products manager Drew Robinson said his utility is fascinated by the opportunity to experience the solar blockage and learn what it can.

“We’re ready for it, and we’re excited,” he said.

KCP&L will be focused on load changes along the grid as well as studying transformer operations and any potential switching problems. He expects a “soft decline” in solar load as the eclipse happens with a gradual increase as the moon and sun separate.

“We’ll see how automation handles it,” Robinson added.

All of those grid experts point out that NASA and even hard-core amateur astronomers have known this solar eclipse was coming for years. Grid planners noted that you don’t get that kind of valuable predictability with wind power or sudden storm outages that interrupt fossil-fueled power sources.

The U.S. Energy Information Agency noted that relatively little solar PV capacity is in the 100-percent path of the eclipse—obscuring sunlight to approximately 1,900 utility-scale PV plants in the country. The North American Electric Reliability Corp. does not expect major impacts for the bulk power system.

“Solar-powered generators centered in the path of totality will be affected the most, as the moon will block all direct sunlight for up to three minutes,” the EIA release reads. “These generators will also be affected to a lesser extent throughout the entire eclipse event, which will last for up to three hours, measured from the onset to the ending of any blockage of direct sunlight. Generators outside of the path of totality will be less affected, depending on how much sunlight is obscured. The path of totality spans the United States, starting in Oregon and moving eastward to South Carolina over the course of approximately 90 minutes.”

Audrey Lee, vice president of grid services for California-based solar services provider Sunrun, believes the fear of the eclipse’s impact on power delivery is more hoopla than justified. Yes, a California Independent System Operator report warns that more than 5,600 MW of Golden State solar could be subtracted during the peak event, but efficiency and demand response efforts will mitigate that imbalance down to a manageable 200 MW or so.

“What is available (from storage and other sources) is plenty to cover that drop,” Lee said. “It seems like a solar eclipse would be like a cloudy day.”

The CAISO report details key statistics of the MW timeline on Aug. 21. For instance, grid-connected solar in that state is forecasted to produce less than 3,200 MW at 10:22 a.m. PT, far below the usual 8,754 MW.

The minute-by-minute drop by MW will be three times faster than usual, some say. It won’t derail the grid but certainly will put  it under unique challenges.

The solar eclipse visible across Europe in March 2015 forced grid planners to respond well in advance. The total obscurity was north of Scotland, but some form of it moved across the continent all the way south to northern Africa, according to NASA’s website.

Germany’s robust solar capacity—then at about 38.5 GW—caused some concern on how to keep the power flowing at an uninterrupted pace. But the Germans embraced it, calling the eclipse a “stress test” on the nation’s fast moving Energiewende movement toward renewables. They carefully balanced output from gas-fired and pumped-hydro plants to deal with the fluctuation from 14,000 MW solar to 7,000 MW and eventually back up to as high as 20,000 MW.

In Italy, grid operator Terna took another approach in reducing its risk by de-linking all of its solar PV plants above 100 kW for the day to avoid severe fluctuations on the system, according to reports.

Sunrun spokesman Andy Newbold pointed that Europe planned for some 80,000 MW of lost solar generation with its 2015 eclipse. The real outcome was about 1/20th of that, yet the event underlined the growing value of solar on the grid.

“It shows how important the sun is, how important solar energy is,” Lee said.

Whatever the U.S. learns, it’ll get one more chance when another total solar eclipse moves from Mexico across the central, upper Midwest and New England in April 2024. Of course, let’s hope none of these are cloudy days.

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The Clarion Energy Content Team is made up of editors from various publications, including POWERGRID International, Power Engineering, Renewable Energy World, Hydro Review, Smart Energy International, and Power Engineering International. Contact the content lead for this publication at

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