Total solar eclipse may reduce US solar power generation by 40 GWh, how five major grid operators are responding

The total solar eclipse on Monday, April 8, will cause a loss of thousands of gigawatt-hours of solar power generation in parts of the United States, and experts say the impact could exceed that of any previous eclipse. National grid operators have been preparing for months and say they can handle the event without affecting reliability. However, they also say that uncertain weather prospects and human behavior changes related to the eclipse make energy demand forecasting more complex.

Although the path of totality only covers some solar facilities in the United States, and cloud cover will affect the extent of the loss, Hugh Cutcher, chief data scientist at Solcast, pointed out: "The impact of this eclipse on solar power generation across the United States will be significant." Solcast is a company that provides solar irradiance data and forecasting technology.

Solcast estimates that during the eclipse, the maximum generation loss across all U.S. grids could be as high as 39.9 gigawatt-hours, with about 16.2 gigawatt-hours coming from rooftop solar.

Despite the reduction in renewable energy, "at the end of the day, we will be fine," said Michael Lee, CEO of Octopus Energy US, a demand response provider operating in the Texas market.

Experts generally expect natural gas to fill most of the solar gap during the eclipse.

"We expect natural gas to be the most likely solution because the eclipse is highly predictable, and every grid has the natural gas infrastructure to meet this demand event, which will exceed last year's, both due to the eclipse path and the increase in installed capacity since October," said Harry Woods, head of marketing at Solcast, in an email.

Lee also said that flexible loads and batteries will also play a role. "Demand response and flexibility can create a low-cost grid," he said. But ultimately, "this event lasts only an hour, and managing millions of thermostats while people are typically out watching the eclipse is a low-hanging fruit opportunity."

California has long been a leader in solar power, but due to the eclipse path, the impact will be relatively mild—potentially losing 5%-6% of daily solar generation, according to Solcast. PJM Interconnection and the Electric Reliability Council of Texas (ERCOT) markets could lose up to 10%-11% of daily solar generation.

But setting aside solar generation loss forecasts, how are grids in different parts of the United States preparing for the upcoming event?

California Independent System Operator (CAISO)

California is no stranger to high-penetration solar or eclipses: last October, the state experienced an annular eclipse that affected the grid from approximately 8:40 a.m. to 9:30 a.m. During that event, grid-scale solar generation declined at a rate of about 82 MW per minute, ultimately reducing output by about 9,600 MW relative to a typical sunny day, according to California Independent System Operator officials.

Monday's eclipse will have a smaller impact than the 2023 event, but the state's rapid solar buildout complicates the situation. During the October 2023 eclipse, CAISO had more than 16,500 MW of grid-scale solar capacity installed, according to the grid operator. Now it exceeds 18,500 MW.

"Although it has been only a little over six months since the 2023 eclipse, we have seen quite a bit of growth in both grid-scale and behind-the-meter solar within CAISO and the energy imbalance market it manages," said Jessica Stewart, senior energy meteorologist at ISO, during a March planning meeting.

Stewart said that during the upcoming event, California will experience eclipse impacts from 10 a.m. to 12:30 p.m., with grid-scale solar capacity declining at a rate of about 79 MW per minute until maximum obscuration. During recovery, solar will rise at a rate of about 84 MW per minute, adding about 6,700 MW to full output.

Officials said the impact of the eclipse on electricity prices will not be known until day-ahead and real-time prices are formed.

ERCOT

The grid operator serving most of Texas said it expects solar generation to be affected for three hours, with the maximum impact occurring at 1:40 p.m., when solar generation will drop to about 7.6% of maximum clear-sky output.

ERCOT has asked all market participants to "monitor operating conditions for the day and adjust current operating plans as necessary," but expects to operate normally during the event.

The grid operator "is working with solar forecast vendors to ensure forecast models account for eclipse impacts," according to a planning presentation submitted last month to the grid operator's Reliability and Operations Subcommittee. "ERCOT will pre-position system conditions as necessary to handle both downward and upward solar ramps and will use ancillary services to meet additional balancing needs."

Lee said this could include the new ERCOT Contingency Reserve Service, which will keep thermal and battery assets on standby. But the primary tool for managing demand changes during the eclipse "remains energy prices."

"As supply changes, real-time energy prices will fluctuate," he said. "This fluctuation is good because it incentivizes more generation to come online and also incentivizes load flexibility to start participating."

PJM Interconnection

Michael Stewart, a senior engineer and load forecasting expert at PJM, said how much solar generation is lost "will depend on the weather pattern of the day." He outlined preparations at a March 7 Operating Committee meeting.

The grid operator is preparing for an 80%-85% loss of generation from 8,200 MW of interconnected resources. There are also about 4,800 MW of behind-the-meter resources that will be affected, thereby increasing grid demand.

Stewart's presentation noted that several variables could affect load, including school closures or eclipse-related travel. It is "not a holiday, but it is also not a normal Monday."

"Closely monitoring weather forecasts, PJM is preparing to dispatch generation as needed to address solar losses, including using reserves and regulation resources as required," the grid operator said in a blog post. "Reserve resources can provide the required backup generation when called upon, while regulation resources can provide energy to help control system voltage and frequency."

New York

In 2017, the New York Independent System Operator lost about 500 MW of generation during an eclipse when the sun was about 75% obscured.

"Today, solar generation across the state is five times what it was then, but cloud cover will ultimately determine how much generation is lost during the eclipse," Mark Taylor, a technology expert and meteorologist at NYISO, said in a statement.

If skies are clear on April 8, the grid operator expects solar generation to be about 3,500 MW at the start of the eclipse. Generation will then decline sharply over a little more than an hour, potentially dropping to about 300 MW, before recovering to 2,000 MW. Between 3:15 and 3:30 p.m., the sun will be completely obscured for up to 3.5 minutes in different parts of the state.

The ISO's solar forecasting tool "predicts the solar generation that can be expected throughout the day and helps our planners and operators determine the resource requirements to meet expected demand," the operator said in a blog post. "If unexpected conditions arise and solar resources do not generate as initially forecast, NYISO's operations team must find ways to make up the difference."

Midcontinent Independent System Operator (MISO)

The last time an eclipse path crossed the Midcontinent Independent System Operator (MISO) region was in 2017, when the grid had only 100 MW of solar. Today it exceeds 5,000 MW.

"We do not expect any reliability-related issues," MISO said in an April 2 blog post.

The eclipse path crosses MISO's southern and central regions, with two control centers located within the path of totality. The grid operator expects "darkness changes" to last from 1:30 p.m. to 4:25 p.m. On a clear day, the grid operator said generation could drop by about 4 GW over 90 minutes, followed by a rebound of 3 GW.

While forecasting electricity demand for a typical day is "usually straightforward," MISO noted that Monday "will not be a typical spring day." School closures, changed business hours, busier hotels and restaurants, and potentially cooler temperatures due to darkness "make accurately forecasting electricity demand for the day more challenging," the grid operator said.