Illustration: Sam Woolley/Gizmodo

For about three hours on August 21st, power grid operators across the United States will be confronted with a sudden drop in available electricity, owing to the first coast-to-coast solar eclipse in nearly a century. Power disruptions are not expected, but only because measures are being taken to make up for the sudden energy shortfall. Here’s the amount of solar power the US is expecting to lose and what grid operators are going to do about it.

The last time Americans saw an eclipse like this was in 1918, and much has changed since then—especially how we get our energy. We’re in the midst of a green energy revolution, where more traditional sources like coal, gas, hydro, and nuclear are slowly being replaced by wind, solar, and geothermal. Solar in particular has taken off in the US, with total solar capacity increasing 8,500-fold in the past 17 years. Today, approximately 1.26 percent of all energy consumed in the United States comes from solar, the primary drivers being the states of California, North Carolina, Arizona, Nevada, and New Jersey.

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Grid operators in the US are already familiar with solar power disruptions due to changes in cloud cover, intense, rainstorms, and seasonal shifts in the length of the day. But for a few critical hours on August 21st, the Moon is going to spoil the solar power show. Skies across the continental US will darken from 11:35 am ET to 2:30 pm ET, most prominently within the 14 states along the path of totality—the stretch along which the Moon will completely obscure the Sun. Even areas outside the path of totality will experience diminished sunlight, creating a challenge for power companies working to keep the lights on.

The eclipse may only last for a couple of minutes at any given location, but grid operators across the US are bracing for the effect.

Image: NASA

To avoid power disruptions, grid operators will have to perform a delicate juggling act, transporting energy across the country to make up for the temporary shortfall in solar-dependent regions, while carefully monitoring the ebb and flow of available solar energy as the Moon drifts across the surface of the Sun. Thankfully, we’d had ample time to prepare—it’s not like we didn’t know this eclipse was coming.

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Grid operators also have a precedent to work with: Back in 2015, a similar event happened in Europe. The solar eclipse of March 20, 2015, caused a “great deviation in the amount of solar generation that was available before, during, and post eclipse,” forcing the intense “coordination of primary, secondary, and tertiary reserves across Europe within a reduced time frame,” according to a policy brief produced by Solar Power Europe in the wake of the celestial show. In Germany, the eclipse caused solar power output to temporarily drop from 14 GW to 7 GW, compared with a 38 GW solar power capacity.

A section of the Nellis Solar Power Plant in Nevada. (Image: US Army)

“The August 21 eclipse is a multi-state event because solar or photovoltaic resources are an increasingly significant part of the electricity generation mix throughout the United States,” said Martin J. Coyne, Communications Coordinator for North American Electric Reliability Corporation (NERC), in an interview with Gizmodo. “The capacity of solar or photovoltaic resources for generating electricity across the United States grew from five megawatts in 2000 to 42,619 MW in 2016.”

Coyne said the impact of an eclipse on the bulk power system depends on several factors, including the event’s duration and timing.

“For most states, the timing of the eclipse is when electricity demand is below the morning [between 6:00 to 9:00 am] and evening peaks [between 4:00 and 8:00 pm],” he explained. “The greatest need for grid operators to plan for the availability of additional generation are in California because it has more solar capacity than any other state and in North Carolina because the eclipse occurs there near the evening [demand] peak.”

If the eclipse occurred at the peak hour in each state, which it is not going to do, the expected impact to solar resources would be between 5,534 MW and 6,329 MW, said Coyne. For comparison, 6,000 MW is the amount of power needed to run six million homes, or a city the size of Los Angeles.

The impact of the eclipse will vary according to region, with the band known as the Path of Totality featuring nearly 100 obscuration of the Sun for a brief period. (Image: NERC)

Indeed, California is expected to bear the brunt of the eclipse’s power disruption, even though the state lies outside the path of totality. California currently has six times more solar than any other state, and its largest independent systems operator, CAISO, has been preparing for a drop in solar generation since 2016.

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In an emailed statement to Gizmodo, the CAISO said the total solar eclipse will “pass over the Pacific Northwest affecting the California ISO solar resources supplying power to the grid. The eclipse is expected to occur from 9:02 am to 11:54 am, with the moon obscuring 58 to 76 percent of solar rays, depending on the resource location, and causing a loss of 4,194 megawatts (MW) of large scale solar electricity.”

The company added that when it factors in folks joining the grid due to a loss of rooftop solar, the net demand, or load, “is anticipated to increase to about 6,008 MW during the eclipse, a gap that will need to be filled using resources other than solar generation.”

CAISO estimates that grid-connected solar production will fall, or ramp down, from 7,337 megawatts at 9:00 am PST to 3,143 megawatts at 10:22 am PST. Shortly before noon, power will ramp back up to 9,046 MW. On a typical day, downward ramping rates can reach 29 MW/minute, but on August 21st, CAISO is expecting a ramp-down rate of 70 MW/minute once the eclipse starts, and a ramp-up rate of 98 MW/minute as the eclipse passes.

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Needless to say, California won’t be the only state facing these issues. NERC projects big ramping swings in North Carolina, Utah, Arizona, and Texas. Even Ontario can expect a noticeable swing.

There are three main electrical grids in the United States and parts of Canada. (Image: NERC)

“Specific states with large amounts of photovoltaic resources are expected to experience an increase in load and possible ramping and balancing concerns,” noted a NERC white paper about the upcoming eclipse. “It is recommended that all states secure non-photovoltaic resources for system operation during the 2017 total eclipse.”

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CAISO has been working with utilities, advising them to prepare for the brief energy shortfall, and the rapid decrease and then increase in solar generation, which is expected to stress the system. It’s asking grid operators to procure additional energy from gas-powered plants, hydro-electric, nuclear, and so on. Utilities are also being asked to tap into the Energy Imbalance Market (EIM), in which members pool their available electricity, and then dispatch it where needed. In regions affected by the eclipse, participating EIM utilities will have about 866 MW of solar to work with, and because members will be affected by the eclipse at slightly different times, they have some temporal wiggle when shuttling this surplus energy around.

The long-and-the-short of all this is that most Americans won’t even notice that anything out of the ordinary is happening in the background. Aside from the total eclipse, it’ll be just a normal day as far as power generation is concerned.

“Blackouts are not expected,” said Coyne. “The grid has built-in redundancies that include a variety of other fuels such as coal, hydro, natural gas, nuclear, and wind. Grid operators carry spinning reserves [i.e. extra generating capacity that’s available by increasing power output of generators that are already connected to the system] to account for load fluctuations and contingencies on a day-to-day basis.”

Map displaying solar energy potential across the United States. When it comes to solar power, we’ve only scratched the surface. (Image: NREL)

The next major US eclipse will occur on April 8, 2024, at which time Texas is expected to take the biggest energy hit due to solar industry growth projections and the path of that eclipse, which will cut a swath right through Texas. By that time, our reliance on solar energy will likely be much greater than it is today, placing even more strain on the grid. As we transition to this clean form of energy, eclipses are poised to be increasingly disruptive events.

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But that’s not to say solar isn’t here to stay—it most certainly is. And if anything, this event will serve to reinforce the introduction of solar as a safe, clean, and reliable energy source. The eclipse on August 21 will appear as a noticeable blip, but one that won’t destabilize the grid (contrary to the concerns of DOE secretary Rick Perry). We’ll just have to plan for these highly predictable—but beautiful—annoyances.

[NERC, CAISO, Space]