Brunch tomorrow in Texas will take place under the eye of Sauron. From about 10:20 am local time in San Antonio, the sky will begin to darken with an annular solar eclipse, in which the moon crosses directly in front of the sun at a time the satellite is especially distant from the Earth. From an earthling’s perspective, the moon will be too small to blot out the sun entirely, leaving fiery tendrils at the edge of a deep black disk. About 10 percent of the sun will manage to flicker past.
For those in charge of converting solar radiation into electricity, that remaining fraction provides little comfort. From California to Texas, grid operator estimates indicate that more than a third of the country’s solar capacity—enough to power about 20 million homes—will be unavailable at some point during the three-hour celestial event tomorrow, presenting a test for electric grids.
The challenge is twofold. Solar power is now far more dominant than it was in 2017, the last time a solar eclipse traversed the continental US. (That was the one where then president Trump stared directly at the sun.) In California, solar generation capacity, including panels on private rooftops, has doubled since then, and the sun now regularly provides more than half the state’s midday needs. Elsewhere, growth has been even stronger, albeit from a lower starting point.
Saturday’s eclipse will take a southerly route across the US, starting on the West Coast in Oregon, and head straight across the nation’s Sun Belt. The entire state of California will fall under partial shadow, dimming solar output to less than a quarter of its typical capacity on a sunny October day, according to the California Independent System Operator (Caiso), the organization that runs the state’s grid. Minutes later, the shadow will reach Texas, where local grid operators forecast output to plunge to 13 percent of its “clear sky” maximum. In between, the eclipse will traverse the additional solar hot spots like Arizona and Nevada. “We’re poised to take much more of a direct hit from this one,” Jessica Stewart, a senior energy meteorologist at Caiso, warned solar providers at a workshop earlier this fall on how they could prepare for the eclipse.
Despite the projected impact of tomorrow’s celestial show, grid officials in those states say that any and all mood lighting tomorrow should come from natural phenomena, not blackouts. Scott Aaronson, a senior vice president of security and preparedness at the Edison Electric Institute, a group that represents investor-owned electric companies, wrote in an email that its members are aware that solar “is intermittent and cannot always be relied upon,” and that the grid is well-prepared to handle the weekend event. Concerns have also been tempered by the crisp October air—temperate enough to reduce demand for both AC and heating—and the eclipse’s arrival on a Saturday, when human activity quiets to a relative murmur. But grid operators and utilities have spent months quietly preparing for the event.
The sun, of course, burns steadily, but weather and other inconveniences turn it into a rather fickle energy source. Wildfire smoke can creep in, or a hurricane, like it did in California in August. Sunset provides a daily rehearsal for losing the light. But an eclipse dims the sun much faster and can do it at an inconvenient time of day. Managing the contribution of renewables to the grid is a delicately balanced seesaw. Excess energy from the midday surge from solar helps charge up batteries that prepare the grid for leaner evening times—and now often come to the grid’s salvation during heat waves. Natural gas plants are used to keep electricity flowing through the night.
To cope with the eclipse, grid operators need to line up enough alternative power sources to fill tomorrow’s gap. The lineup will be decided day-of, but will potentially include firing up natural gas plants earlier in the day, as well as tapping grid-scale batteries that have been charged overnight.
Perhaps even trickier is smoothly getting all the solar generation capacity back online a few hours later, when the sun is even higher in the sky, without discombobulating the grid. At that point, the grid will experience ramp rates—the rate of increase in power coming online—that exceed anything the grid typically experiences, says Amber Motley, director of short-term forecasting at Caiso. At that point, the ramp rate will be 120 megawatts per minute, 10 times the norm for that time of day.
Choreographing that ramp-up is a high-wire act: Too slow and supply may fall below demand, meaning outages; too fast and the grid becomes overloaded, making it difficult to maintain the proper frequency. Caiso is confident they can handle it, Motley says. But doing so requires careful coordination between hundreds of power plants.
The sudden loss of sunlight has second- and third-order effects too. Last month, forecasters at the US National Oceanic and Atmospheric Administration projected temperatures would decrease by up to 10 degrees Fahrenheit (5.5 Celsius) in areas under the full shadow of the annular eclipse. Reduced solar radiation in the morning means less air turbulence, potentially calming winds by as much as 6 miles per hour. That could reduce power output from wind turbines by about 10 percent.
For much of the western US, the electric grid now has more connectivity between different states than during its last eclipse workout in 2017, Motley says. That allows states to help each other out, tapping cheaper resources and redirecting power along less congested transmission lines. That flexibility is valuable during rare events, like storms or the eclipse tomorrow. “There are a lot of benefits from the diversity of those resources and also the timing,” she says. “The eclipse’s obscuration impacts are not hitting all of us at the same time.”
One state that can’t tap those communal benefits is Texas, which operates its own grid. There, the Electric Reliability Council of Texas, or ERCOT, has overseen a similar surge of renewables and batteries in recent years, but balance problems—most notably in 2021, when millions in the state lost power during a winter storm—have continued. Last month, amid a late-summer heat wave, the grid went into high alert when energy reserves fell perilously low, and the state is scrambling to procure more generation in time for winter.
A statement from ERCOT says that it should have plenty of energy in reserve during the eclipse, citing the cooler October weather this weekend, with the heat expected to break just in time in cities like Houston. For Texans, the event will be a rehearsal for an even bigger test on April 8, 2024, when yet another eclipse will cross directly over the state and head toward the northeast US. This time, the moon will be close enough for the eclipse to be total, obscuring the sun by 100 percent.
