Early April is typically a kind of goldilocks moment for solar power. Days are getting longer but the weather is still mild, meaning that higher solar power generation isn’t entirely eaten up by increased demand due to air conditioning.
But that all depends on the sun actually shining.
Monday’s solar eclipse took a big chunk of power off the grid. Since 2017’s eclipse, solar power generation has increased substantially, both locally (think rooftops) and at utility scale (think massive fields of solar panels). In 2017, the U.S. had around 35 gigawatts of utility-scale solar capacity, a figure that had increased to an estimated 95 gigawatts by the end of 2023.
While total solar eclipses are rare (the next one to hit the lower 48 isn’t expected until 2044), the challenges they present to grid operators may be part of the new normal. With vastly expanded renewable energy generation comes a greater degree of unpredictability, as a growing a portion of the generation fleet can drop off the grid due to weather and climate conditions — like, say, clouds of smoke from a wildfire — that cannot be precisely predicted by 17th century science.
Grid operators were confident they’d be able to manage through the eclipse without any reliability issues, and what actually transpired mostly confirmed their forecasts. In Texas, solar power production shrunk from around 13.5 gigawatts at noon, making up 27% of the grid’s electricity supply, to a mere 0.8 gigawatts at 1:30 p.m. Things did not go as well for the Midcontinent Independent System Operator, however, which includes a swath of the middle of the country from Minnesota to Indiana to Louisiana. Solar output was estimated to drop from around 4 gigawatts at 1 p.m. Central time to 2 gigawatts an hour later, according to Grid Status. Instead, output dropped to around 300 megawatts, causing real-time prices for power on the grid to spike.
ERCOT’s fuel mix from Monday, April 8, including during the solar eclipse.ERCOT
Overall, the U.S. Energy Information Administration estimated that some 6,500 megawatts of solar generation capacity would be fully obscured during the eclipse, which would “partially block sunlight to facilities with a combined 84.8 GW of capacity in an even larger swath of the United States around peak solar generating time.” Some 40 gigawatts may have come off the grid, enough power for about 28 million homes, according to a release from Solcast, a solar forecasting company.
By comparison, during the 2017 eclipse, solar power loss at its peak was between 4 and 6.5 gigawatts and the total loss of power was around 11 gigawatts, according to the National Renewable Energy Laboratory.
In states like Texas, the main effect was on utility-scale production of solar, but in the Northeast and parts of the mid-Atlantic and Midwest, there was also a related problem: Behind-the-meter solar fell off, too, thus requiring the homes and businesses that generate power for themselves in the middle of the day to get more power from the grid, increasing demand on the grid at a time of low supply.
New England has seen immense growth in rooftop solar, and solar production was expected to fall by “thousands” of megawatts, according to ISO New England, while the New York Independent System Operators expected to lose 700 megawatts of behind the meter solar.
During the 2017 eclipse, the National Renewable Energy Laboratory found that “the burden of compensating for the lost energy from solar generators fell to the thermal fleet,” i.e. natural gas, along with some increases in coal and hydropower production.
Since then, the coal fleet has shrunk, thus putting more of the burden of responding to Monday’s eclipse onto gas and hydro, but the basic logic still applies. “Grid operators are expected to rely on natural gas to ensure stability and meet the household demand spike across national grids, as was done during the previous eclipse in 2023 in California and Texas,” according to Solcast. As the sun was dimming in Texas, natural gas generation rose from 18.7 gigawatts to 27.5 gigawatts.
Something else that’s changed since 2017: batteries. By the end of 2023, Texas had installed 5.6 gigawatts of grid storage, most of it providing so-called “ancillary services,” power sources that can respond quickly to immediate needs. ERCOT, the electricity market that covers most of Texas, said in a presentation back in February that it would rely on these ancillary tools to get through the eclipse, and once again, it was right. Power from batteries on the grid got up 1.4 gigawatts during the eclipse.
Editor’s note: This story has been updated to reflect the actual effects of the eclipse on U.S. power generation.
ERCOT’s fuel mix from Monday, April 8, including during the solar eclipse.ERCOT
IEA