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Grid Operators Provide Details on Potential Impact of April Solar Eclipse

Grid operators across the U.S. are detailing their plans to respond to a total solar eclipse that will occur on April 8, as well as projecting the expected impact of the eclipse on their grids.

ISO New England

ISO New England said it is taking steps to prepare the region’s power system for next month’s total solar eclipse, which is expected to reduce solar energy production by thousands of megawatts during the afternoon of April 8.

ISO New England said its operators are familiar with managing the grid through situations where electricity demand dips and spikes, such as previous solar eclipses.

To help ensure reliability and keep supply and demand in balance, ISO forecasters plan ahead and chart out expected energy demand from the grid for each hour a day in advance. These daily forecasts take a number of factors into account, including the season, day of the week, time of day, weather, and special circumstances such as an eclipse, it said.

In the United States, the eclipse’s path of totality, where the moon will completely block out the sun, spans from Texas to Maine, enveloping northern Vermont and northern New Hampshire from 2:15 to 4:40 p.m.

ISO New England is collaborating with local utilities and neighboring grid operators to evaluate the expected conditions and potential impacts.

According to NASA, all parts of New England will see at least 80% of the sun blocked by the moon during the peak of the eclipse.

The vast majority of New England’s solar power comes from small-scale, distributed systems connected directly to retail customers or to local utilities and not the regional power system operated by the ISO. These systems reduce the amount of demand on the grid when the sun is up, ISO-NE said.

Unlike the sun setting at the end of the day, solar production is expected to drop off fairly rapidly over the course of the eclipse and will need to be replaced by other sources of power generation. The exact amount of additional generation needed will depend on cloud cover that day. Further, solar production will ramp back up as the eclipse concludes, meaning that additional generation will need to be carefully reduced to maintain system balance, ISO-NE said.

“Even a partial eclipse can have a major and measured impact on the power system,” it noted. For context, during the partial eclipse on October 14, 2023, the sun was only covered by 10% to 20% and resulted in a drop in solar production of several hundred megawatts.

Though not anticipated, ISO system operators have a number of tools available to handle any supply deficits caused by extremely high demand or a sudden loss of other resources.

California ISO

“On Monday, April 8, 2024, a partial solar eclipse will cross over the western United States, impacting both grid-connected and behind-the-meter rooftop solar generation in the California ISO balancing authority and its real-time Western Energy Imbalance Market (WEIM),” the California ISO said.

“With the electricity grids in the West more interconnected than ever, the CAISO has studied the potential effects of widespread solar generation loss during the three-hour eclipse, identifying risks and solutions, and coordinating with WEIM participants, the Reliability Coordinator West (RC West), and other balancing authorities and stakeholders to maintain grid reliability,” it said.

The California ISO has posted its Solar Eclipse Technical Bulletin analyzing impacts from the April 8 event.

The April eclipse will affect the ISO and WEIM differently than the annular solar eclipse that occurred on October 14, 2023, the grid operator said.

An annular eclipse obscures most of the sun except for a halo of sunlight around the edge of the moon’s dark disc. The April eclipse will be total, meaning that the entirety of the sun will be blocked by the moon, but it will travel a different path, much farther to the east.

The October 2023 eclipse’s path of annularity passed across eight Western states from Oregon to Texas.

In contrast, the April eclipse’s path of totality will not pass across the West. “Its effects on the WEIM and California will be far less than in October. In October, the ISO weighed the huge buildout of utility-scale solar between the last eclipse in 2017 and the 2023 eclipse,” CAISO said.

Since October, there has been some growth in grid-scale solar but nowhere near as much as between the 2017 and 2023 eclipses. During the October 2023 eclipse, there were more than 16,500 MW of grid-scale solar installed capacity throughout CAISO.

There are now more than 18,500 MW. At the same time, behind-the-meter rooftop solar capacity has increased from 14,350 MW in the CAISO BAA to 15,770.

CAISO said the April 2024 eclipse will be less impactful than the 2023 eclipse because of the eclipse’s trajectory.

“Solar obscuration across California and the West will be far less than in October, and the impacts to load and grid-scale renewables will be less significant.”

CAISO said this is true even though CAISO grid-scale and rooftop solar have grown by over 2,000 MW and 1,420 MW respectively since October 2023, and WEIM grid-scale and rooftop solar have grown by 1,870 MW and 445 MW since October.

For the CAISO Balancing Authority Area, the eclipse will start to impact grid-scale solar and load at 10:00 a.m. on April 8, reaching maximum impact at 11:15 a.m. before returning to normal conditions at 12:30 p.m. Precise timing and levels of obscuration will depend on geographic location relative to the eclipse path.

In comparison, the October eclipse began around 8:00 a.m. in the CAISO BAA, reaching peak conditions at 9:30 a.m. and ending around 11 a.m. The CAISO BAA has approximately 18,500 MW of installed grid-scale solar capacity and 15,770 MW of rooftop solar capacity.

Obscuration from the eclipse will reduce generation from all solar resources. The change in grid-scale solar generation and gross load is greatest on the eclipse return.

Grid-scale solar generation will decrease by 6,349 MW from the start of the eclipse to maximum impact and increase by 6,718 MW on the eclipse return. Gross load will increase by 2,294 MW from the start of the eclipse to maximum impact and drop by 5,159 MW from the eclipse maximum to the eclipse end. Net load ramp rates on the eclipse return are larger than from the eclipse start to maximum impact.

At the eclipse start to the period of maximum impact, the net load ramp rate will increase by an average of +115 MW per minute and decrease by -150 MW per minute on the return. The ramp-up rates are expected to be similar to the October 2023 eclipse ramp rates, which exhibited an average +100 MW per minute net load ramp rate. Ramp down rates are expected to be slightly steeper compared to the October 2023 eclipse, which exhibited -118 MW per minute net load ramp.

WEIM regions will see varying eclipse impacts on solar generation and load depending on levels of installed grid-scale and behind-the-meter rooftop solar.

The Central region has the largest amounts of installed grid-scale solar capacity, while the Desert Southwest region has the greatest total installed BTM solar capacity. The Central and Desert Southwest regional loads are forecast to increase by 4.3% and 19.4% during the upcoming April 2024 eclipse ramp-up period, respectively.

During the October 2023 eclipse, these regions saw 5.2% and 12.9% increases during the eclipse ramp-up period. The ISO will coordinate with RC West, utility distribution companies, and WEIM entities to ensure stable market operations to support reliable operations on the eclipse day.

CAISO and WEIM renewable scheduling coordinators “should ensure appropriate eclipse impacts in their forecasts. Grid-scale solar resources will use special procedures to manage eclipse ramping requirements. The specialized solar procedures include the potential to limit the solar generation ramp rate during the eclipse return and to utilize an Operating Instruction (OI) for the fleet to ensure solar resources follow their Dispatch Operating Target (DOT) and linearly ramp from Dispatch Operating Point (DOP) to DOP,” CAISO said.

The California grid operator said that much of the success of the 2023 eclipse event was due to the accuracy with which solar generation was forecasted in the day-ahead timeframe, which limited the need for real-time re-dispatch.

“Due to the expected impact on solar resources, it is critical that solar resources within CAISO and the WEIM footprint account for the eclipse impacts in their market submittals,” CAISO said.

The ISO said it will ensure coordination with hydro and battery resources to safeguard availability to assist with large ramps expected on the system during the eclipse.

Similar to the growth of solar since 2023, CAISO has seen some growth in battery resources to assist with the faster ramping needs on the system.

The ISO plans to use battery and hydro resources during the solar eclipse.

The ISO will procure additional operating reserve requirements to assist in the eclipse. During the 2023 eclipse, additional reserve procurement helped grid operators handle resource deviation in addition to large ramps on the system.

For 2024, the ISO will procure additional operating reserves to assist with the expected change in solar generation and to offset potential cloud cover. To get the most benefit while minimizing risk, greater coordination and preparation across utilities is required throughout the RC West and WEIM footprints and the CAISO balancing area.

“The ISO remains committed to providing details on potential impacts and actions to allow entities to prepare for the eclipse, as well as support reliable operations throughout the event,” it said.

A stakeholder call on the bulletin is scheduled for Monday, March 11. You must pre-register to receive a link to join the event.


On April 8, 2024, a total solar eclipse will pass over the state of Texas from the Southwest to Northeast direction, the Electric Reliability Council of Texas told Public Power Current.

The event will impact solar power production in the region between approximately 12:10 p.m. and 3:10 p.m. CDT. 

“ERCOT is working on forecasting models to reflect solar generation on the grid during the eclipse. ERCOT does not expect any grid reliability concerns during the eclipse. ERCOT will use all available tools to maintain grid reliability and will continue to monitor conditions and keep the public informed through our communications channels,” it said.

PJM Interconnection

Meanwhile, staff at the PJM Interconnection reported on the upcoming eclipse at a March 7 Operations Committee meeting, which covered potential impacts to generation and load based on possible weather conditions that day.

At the next committee meeting on April 4, staff will do a deeper report out based on actual forecast data for expected temperature and cloud cover on April 8, allowing the operations approach to be more refined for the event, said Dan Lockwood, a PJM spokesperson.

PJM will also publish a story on preparations on its news site, Inside Lines, “as we get closer to the eclipse,” said Lockwood.

“In general, PJM prepares for an event like this by simulating the possible reduction of solar resources in areas of the footprint in the path of the eclipse. That analysis determines if any adjustments will need to be made to PJM’s operations on that day. PJM is also prepared to schedule and deploy additional generation and regulation resources, if necessary, to account for possible shifts in load,” he said.

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