A tale of two grids: how CA and TX generation responded differently to the April 2024 solar eclipse

On April 8, 2024 the contiguous United States experienced its second total solar eclipse of the 21st century. The first happened in 2017; the next won’t happen for another two decades. No shortage of digital ink was spent covering the run-up to — and post-mortem analysis of — the eclipse, and especially how it impacted solar PV generation across the country.

Coverage ranged from the measured (“Darkness from April's eclipse will briefly impact solar power in its path. Experts say there's no need to worry,” noted USA Today) to the dramatic (“The solar eclipse is a critical test for the US power grid,” declared Vox) to outright fear-mongering (the New York Times and many others debunked myths that the eclipse would cause the grid to fail).

In practice, grid operators as well as government agencies such as US EIA and NREL were well-prepared for this year’s Great North American Eclipse, as it’s become known. But exactly how the nation’s grid operators handled the predicted drop in solar power generation differed significantly, which is what we’re examining more closely in this blog post.

In California, batteries that charged on excess renewable energy backfilled solar’s slump

Across the Western Interconnection (WECC) — which includes all or part of 14 U.S. states — the percent of solar obscuration ranged from 20% in the Pacific Northwest (farthest from the path of totality) to 80% in the southeast corner of New Mexico. Across all of WECC, NREL estimated that the maximum reduction in solar PV generation would reach 45%, although that varied significantly by proximity to the eclipse path.

In California, the impact ranged from ~30% for utility-scale solar farms in the central part of the state to 50+% for solar in southern California. Statewide on April 8, CAISO reported that solar generation peaked that morning at close to 14.5 GW, plateaued around 12.4 GW through most of mid-morning, then fell a further ~27%, bottoming out at ~9.1 GW around 11:15 am. By 12:15 pm — with the eclipse over — solar generation had rebounded to 14+ GW.

That much of the story has already been well-reported, but at least two other interesting things happened in tandem.

First, through the hours of the eclipse, solar curtailment on CAISO’s grid all but disappeared. In the hour before the eclipse, California discarded more than 2.5 GWh of solar energy while simultaneously charging energy storage.

Second, battery energy storage — which normally charges during daytime periods of solar excess generation in preparation for California’s evening peak — flipped from charging at nearly 2.6 GW into discharging at 2.7 GW in less than an hour. In doing so, storage almost entirely backfilled the midday solar slump from the eclipse. Meanwhile, natural gas — which usually sleeps during the day awaiting the evening ramp — barely registered a change in generation. After the eclipse, energy storage resumed charging in preparation for the evening peak.

In Texas, natural gas illuminated the darkness

The dark path of this year’s eclipse passed straight through the heart of ERCOT solar country, where NREL forecasted up to a 93% drop in peak solar PV output. ERCOT data confirm that reality matched expectations: solar generation plummeted from ~13.8 GW at 12:15 pm local time to just 0.8 GW a short 45 minutes later at 1:30 pm, a 94% reduction. By 2:45 pm, solar was back up to 13.7 GW. Solar’s generation profile that day looked like a narrow-waisted hourglass tipped on its side, going from 27.6% of ERCOT generation to 1.7% and back up to 27% in the span of just two hours.

But unlike in CAISO — where batteries were the chief responding resource — in ERCOT natural gas stepped in to meet demand, ramping up from ~19 GW to 27+ GW, then quickly tapering back to ~18 GW. Energy storage made a smaller, incremental contribution of ~1.4 GW during the peak of the eclipse, but gas-fired generators dominated the response.

Across the Eastern Interconnection, the story was much the same as in Texas. In PJM — where totality passed through Ohio and then western Pennsylvania — natural gas backfilled solar’s temporary dip. That motif repeated in NYISO, and then ISO New England. In New York and New England, behind-the-meter solar — rather than utility-scale solar — was the protagonist. In each case, though, the grid response followed suit, with natural gas stepping in.

Conclusion

The response to the eclipse can be seen as a microcosm of how grids are managing the transition to renewables and their predictable variability.

Places like California are using energy storage (usually charged on excess renewable energy) to fill the gaps in the fluctuations of wind and solar energy (not to mention sudden disruptions in fossil-fueled thermal power plants). In grids like Texas and the Northeast, where there is not yet considerable excess renewable energy or sufficient energy storage, fossil natural gas plants are used to make up the difference.

Maintaining grid reliability while also minimizing electricity-related emissions requires a detailed understanding of how power plants, energy storage, and load flexibility can all participate in a choreographed dance to support the grid’s real-time needs for supply / demand balance.

Hero image of the 2024 solar eclipse passing over the Washington Monument in Washington, DC, by NASA/Bill Ingalls. Used with permission via CC BY-NC-ND 2.0 DEED.