The coming total solar eclipse on Monday, Aug. 21, will obscure at least a sliver of sunlight harnessed by every solar power plant in the United States, temporarily depriving roughly 1.4 million generators of much, if not all, of their energy source. Grid operators say they are prepared, but there could be surprises.
The path of totality, spanning Oregon to South Carolina, will fully shadow only 17 utility-scale photovoltaic facilities, where the moon will block all direct sunlight for up to three minutes and affect the plants' output for up to three hours, according to an Aug. 7 briefing from the U.S. Energy Information Administration. Of the country's approximately 21,750 MW of installed utility-scale PV, all but 100 MW is in areas where the sun will be obscured at least 50%, EIA's data shows. Of that, 10,210 MW will lose at least 70% of solar radiation and 6,280 MW at least 80%. About 4,000 MW, mainly in North Carolina and Georgia, will be 90% or more obscured.
As the moon's long shadow approaches, grid operators are reassuring Americans that their lights will stay on. The California ISO, for instance, which hosts nearly 10,000 MW of utility-scale solar that serve up to 40% of its load, the most of any U.S. transmission operator, expects a "sunny outcome for the grid during [the] eclipse," the ISO said in an Aug. 7 statement.
"We are confident we have the capacity to replace the 6,008 MW, including 4,200 MW of utility-scale solar, that will stop producing," spokesman Steven Greenlee said in an interview. The ISO will cover demand between about 9 a.m. and noon PT as the shadow moves across the state. The grid operator will look to hydropower, natural gas plants and neighboring systems for replacement power. The California Public Utilities Commission, meanwhile, has launched a consumer-focused conservation campaign to reduce demand that day.
Duke Energy Corp. in North Carolina, which hosts the second-largest PV fleet in the country, expects to lose up to 2,300 MW of solar generation in its service territory between around 1 p.m and 3 p.m. ET, a Duke Energy director said in an Aug. 1 blog post. PJM Interconnection, North America's largest power grid operator, anticipates the loss of up to 2,500 MW of solar, including 500 MW connected to its 13-state system and 2,000 MW of rooftop installations.
"Certainly, this is an unusual solar event, but as far as potential impacts to the grid, PJM and its members are prepared," PJM President and CEO Andrew Ott said in a July 31 statement. "While this is an anticipated event, we routinely plan and prepare for unpredictable events or things that can't be forecast far in advance, such as severe storms and heat waves."
In an April report, the North American Electric Reliability Corp. found the eclipse was "unlikely to cause any reliability issues to the North American bulk power system." But there is "greater reliability risk created by solar eclipses," it cautioned. To be sure, as Aug. 21 nears, PJM, Duke, the California ISO and other grid operators will all have a close eye on the weather, which will influence how much solar ultimately drops off during the eclipse and returns after it passes, as well as the level of demand during that time. That, in turn, will determine how much power they need to fill the gap.
"The big caveat is what will the weather be?" California ISO's Greenlee said. "Could there be a heat wave that drives up demand or unrelated outages because of wildfire? We will be watching the weather and temperature very closely. Each degree can mean hundreds of megawatts of demand."
Some analysts, however, question the ISO's planning. "In comparing forecasts, it is evident we anticipate load [not including] solar on August 21 will be stronger than CAISO is currently expecting," wrote Genscape analysts in a Aug. 7 blog post. "Should the ISO move forward with the assumptions they've outlined, an average August load day would drive a considerable under-commitment of resources and increase the likelihood of price volatility."
Asked about such a possibility, Greenlee said, "We can't speak to that kind of analysis or conclusion." Based on what the ISO has learned from an eclipse that hit Germany in 2015, it is lining up additional reserves, he added. While the event did not destabilize Germany's solar-rich system, "they did not have enough reserves, so we will have about 200 MW more than we normally have."