“An eclipse is obviously not something we see every day, but this is going to be a good exercise for us,” said Randy Wheeless, a spokesman for Duke Energy, which expects solar capacity in North Carolina to dip from 2,500 megawatts to just 200 on Monday afternoon, affecting roughly 3 percent of electricity generation in the state. “There’s no doubt more solar power is going to come onto the grid in the future, and that does increase the challenge of balancing the grid even on days when there’s not an eclipse.”
Nowhere is this challenge more stark than in California, home to nearly half the nation’s photovoltaic panels. The California I.S.O. already has to handle a steep ramp-up of solar power every morning and a similarly steep ramp-down every evening. Because it can be difficult and costly to quickly switch gas turbines on and off to compensate, the state is getting creative in juggling its fast-growing solar load.
Some of its tactics will be put to the test on Monday. For example, California has agreements with Arizona, Nevada and Oregon to transfer small amounts of electricity between states to balance out fluctuations. The state may need to call on this “energy imbalance market” during the eclipse, Mr. Schmitt said.
In the future, as California plans to increase its share of renewable energy to address climate change, such interstate transfers may prove even more crucial. The idea is that a cloudy day or a thunderstorm (or an eclipse) may knock out solar panels in a few areas, but it is unlikely to knock out solar panels and wind turbines all across the West at once. Spreading out renewable energy across a larger geographical area could help mitigate interruptions.
Creating a fully regional energy market would require further negotiations between states, but Mr. Schmitt sees the eclipse as a way to “dip our toes in the water and see what that looks like.”
California also makes heavy use of pumped hydroelectric storage, in which surplus electricity during the day is used to pump water up a hill. The water can later be released downhill to power a turbine that generates electricity in periods of high demand. The eclipse will help the California I.S.O. test whether its electricity markets are properly set up to call on enough pumped hydropower if needed.
The stakes are high: Some skeptics of renewable energy have questioned whether increasing shares of wind and solar might make the grid less reliable, and the Department of Energy is studying the question. But independent researchers have argued that the nation’s grid can accommodate much larger shares of wind and solar than today’s levels and still withstand fluctuations in the weather. A steady response to the eclipse could bolster those arguments.
Elsewhere in the country, grid operators are hoping the eclipse will help them learn more about how solar power behaves during disruptions, which will allow them to improve forecasting tools that they use to prepare for cloudy days or storms.
For instance, PJM Interconnection oversees the regional power grid that serves 65 million people in the Mid-Atlantic. While solar power still provides less than 1 percent of its supply, PJM is trying to get a handle on the growing number of rooftop solar panels that sit behind home meters, largely hidden to grid operators. In essence, they have to estimate how much rooftop solar power is out there and how it affects daily loads, and the eclipse offers a way to test those estimates.
“I’m actually hoping for a bright, sunny day,” said Ken Seiler, PJM’s executive director of systems operations. A sharp fall and subsequent rise in solar activity “will really help us validate our models,” he said. (For now, PJM expects to lose up to 2,000 megawatts of rooftop solar power during the eclipse.)
Meanwhile, the Electric Power Research Institute, a nonprofit group that performs research for the nation’s electric utilities, will monitor solar farms across the country to see how they respond to the eclipse. Ben York, a senior project engineer at the institute, said the data may prove useful in fine-tuning technologies like solar tracking systems, which allow panels to tilt during the day to maximize sunlight exposure.
Grid operators are already casting an eye toward the next eclipse. While such events may seem rare, a total solar eclipse occurs somewhere in the world roughly every two years. Europe’s grid operators dealt with a total eclipse in 2015 — an event that American operators have studied thoroughly. And another total solar eclipse will carve a swath through the Eastern United States in 2024.
“We’re expecting solar power to be a much bigger part of our footprint by 2024,” said Mr. Seiler of PJM. “And we want to be ready.”