Solar power and the Great American Eclipse of 2024

A total solar eclipse traversed North America on April 8, 2024, on a path from Mexico, across the U.S. from Dallas to northern New England, and into Canada. No total eclipse has covered such a large extent of the U.S. in recent years (a 2017 eclipse was the first in the U.S. in 26 years); meanwhile, an increasing amount of solar photovoltaic power plants have been built. The "Great North American Eclipse" of 2024 thus provided an opportunity to examine how a total solar eclipse affects solar power and the broader grid, using New England as an example.

According to New England's regional grid operator ISO New England, "the April 8 solar eclipse led to a steep and significant decrease in solar energy production, but due to extensive planning by ISO New England operators, the event caused no disruptions to the power system." All of New England experienced at least a partial eclipse where the moon blocked at least 80% of the sun. Parts of Vermont, New Hampshire, and Maine experienced totality.

During the two hours around the eclipse's peak, ISO-NE reports that regional solar production dropped by as much as 4,000 megawatts. 

Chart from ISO-NE (source)

Most of the drop-off -- about 3,300 to 3,500 MW -- came from small-scale, distributed photovoltaic panels connected directly to distribution systems which ISO-NE says "make up the vast majority of solar resources in New England", while grid-connected solar system production dropped by about 650 MW. The timing of the eclipse, relatively late in the afternoon, meant that only about 1,350 MW of solar production returned to the system after the eclipse passed.

ISO reports that its market yielded negative real-time prices as the eclipse approached, followed by a price spike over $100 as the eclipse passed overhead, and then price moderation toward the day's average value.

ISO-NE reports no power system or reliability disruptions from the eclipse, thanks to the system operator's planning for the eclipse, and operations which dispatched natural gas and hydroelectric generators to make up the solar power blocked by the moon. 

California grid prepares for solar eclipse

As a total solar eclipse approaches for North America, California electricity regulators have launched a voluntary demand response program designed to reduce power consumption during the eclipse while solar panel output is reduced.

The eclipse will occur on August 21, 2017, and is projected to reduce solar photovoltaic production in the California ISO region by 4,194 megawatts.  Taking gross load increases and estimated wind production into account, CAISO has been told to expect a net load increase of 6,008 MW during the eclipse.

According to the nation's electric reliability organization, NERC, the August 21 eclipse "is not expected to impact the reliability of the bulk power system."  But as NERC also noted, "As the number of photovoltaic generators on the power system increases, the risk created by solar eclipses to reliable system operations will increase as well."

Now, the California Public Utilities Commission has launched a "Do Your Thing for the Sun" or "Cal Eclipse" program.  On its website, the Commission asks, "While our utilities and grid operator have all the tools necessary to manage the grid during the eclipse, what if millions of Californians stepped in to allow our hard working sun to take a break, rather than relying on expensive and inefficient natural gas peaking power plants?"

The website asks consumers to "Take the Pledge", emphasizing the value of "joining a movement of Californians who are taking action during the eclipse to give the sun a break by saving energy and reducing GHG emissions." According to a two-page FAQ posted on the website, consumers can reduce electricity consumption by turning off electronics when leaving, and permanently decrease electricity consumption with energy efficiency measures.  Actions suggested on the pledge website include replacing light bulbs with LEDs, reducing lighting use and electronics charging, unplugging unused appliances, and increasing air conditioning temperature setpoints by 2-5 degrees.

According to the Commission's FAQ, "There is no reason to anticipate any eclipse-related electric service outages because of the reduced solar generation."

Total eclipses, solar PV and the grid

Utilities and electric grid coordinators are preparing for a total solar eclipse that is projected to temporarily reduce solar photovoltaic generation across parts of North America this summer. 

The 2017 total solar eclipse will be the first in the U.S. in 26 years (since Hawaii 1991), and the first in the lower 48 states since 1979.  While the duration of the total eclipse across the U.S. will be roughly 93 minutes, some areas in its path will experience up to 95% of the Sun being obscured.

The eclipse is projected to affect solar PV generation.  Solar resources occupy an increasing role in the U.S. electric generating portfolio. Between 2000 and 2016, total U.S. solar capacity increased from 5 megawatts (MW) to 42,619 MW.  But as more solar resources are connected to the grid, the potential impact of an eclipse on grid operations may change.

According to a May 1, 2017 presentation to the Board of Governors of the California ISO, the eclipse is projected to reduce solar output in the CAISO region by 4,194 megawatts, while gross load will increase by 1,365 MW.  Taking into account estimated wind production, the presentation projects a net load increase of 6,008 MW during the eclipse.

The ramp rate, or speed at which supply and demand will change, is also a factor.  The eclipse is projected to diminish solar output by about 70 MW per minute as it approaches totality, and about 90 MW per minute on the return.  By contrast, a typical average ramp rate for CAISO might be 29 MW per minute.  Thus the eclipse is projected to call for a greater degree of fast-ramping or flexible resources, compared to typical operating conditions.

But according to international electric reliability organization NERC, the August 21, 2017 total solar eclipse "is unlikely to cause any reliability issues to the North American bulk power system."  NERC documented its findings in an April 25, 2017 white paper, A Wide-Area Perspective on the August 21, 2017 Total Solar Eclipse.  NERC's report identifies California and North Carolina as the states most likely to experience the greatest impact from solar production fall-off from the eclipse. At the same time, NERC recommends "that utilities in all states perform specific studies of the eclipse’s impact of solar photovoltaic power output on their systems and retain necessary resources to meet the increased electricity demand requirements."  In particular, NERC notes that generation and system operators may greater visibility into utility-scale solar projects than into behind-the-meter or distributed solar photovoltaic resources, highlighting the need to model all scales of solar development.

Following the 2017 eclipse, the next total solar eclipse is projected to cross North America on April 8, 2024.