On Sunday night, as a burst of Arctic air swept southward across the Great Plains, power plants in Texas started flicking offline.
Wind generation fell 32% between 9 p.m. Sunday and 3 a.m. Monday local time, according to U.S. Energy Information Administration figures. Coal dropped 13%. And natural gas generation, the cornerstone of the Texas grid, plummeted 25% over that six-hour period.
By the time the sun rose over Texas around 7 a.m., energy demand on the state's primary electric grid had surged to about 71 gigawatts. Texas power plants were only able to muster up roughly 51 GW of electricity, leaving millions without power and shivering in the cold.
It was a different story farther north. Temperatures were even colder in parts of the Southwest Power Pool (SPP), a 14-state electricity system stretching from North Texas to the Canadian border. But generation from coal and gas plants there held steady and even increased. Wind output, meanwhile, followed a normal pattern of declining throughout the day before picking up again later Monday. It was a similar situation in the Midcontinent Independent System Operator (MISO), a 15-state system going from Louisiana to the Canadian province of Manitoba.
Neither MISO nor SPP was able to completely avoid the grips of the cold. Both were forced to sever electricity to customers in a bid to stabilize their grids. Yet those outages were limited in duration and scope compared with what unfolded in Texas.
The differences point to some of the key failures that have forced the Electric Reliability Council of Texas, the state's primary grid operator, to enforce sustained blackouts since Monday. It also highlights the challenge facing the Biden administration and climate activists as they seek to decarbonize America's collection of electric grids.
ERCOT's challenges are several. For one, it is more reliant on gas for electricity generation than its neighbors, which still boast sizable coal fleets.
That matters for two reasons. Gas also serves heating needs, meaning demand for the fuel surges during a cold snap. Coal faces no such competition. It also leaves the state vulnerable to disruptions in gas supplies. Many gas wells also produce water and distillates, which can freeze during cold spells.
"One of the big lessons here is gas is treated as a firm resource, but it is not because it relies on just-in-time delivery," said Alex Gilbert, a fellow who studies energy systems at the Colorado School of Mines' Payne Institute for Public Policy. "For me, looking at SPP and MISO south, there are other planning reasons involved, but they have a more diversified mix and that is definitely helping them."
That's not an endorsement of coal. Most coal plants in the United States are older and utilities are investing less in them now, two factors that increase the probability of forced outages during extreme weather events. Instead, Gilbert argued the United States needs to take a holistic approach to energy planning that includes other factors, such as energy efficiency and development of clean technologies to complement wind and solar.
Another item on that list is planning for extreme weather events, an area where ERCOT has fallen short.
In its most recent winter reliability assessment, the North American Electric Reliability Corp. predicted winter demand in ERCOT would peak around 67 GW in an extreme weather event. U.S. Energy Information Administration data shows that ERCOT demand reached a forecast peak of 75 GW on Tuesday morning.
At the same time, the number of plant outages for ERCOT's grid is far greater than expected. The grid operator forecast nearly 14 GW of plant outages during an extreme winter weather event. Last night, ERCOT officials said some 43 GW has been forced offline. That sum includes plants that were already offline for scheduled maintenance.
The sheer number of plant outages points to the largest difference between ERCOT and its counterparts. The Texas system was simply not prepared for the cold, despite having experienced freezing temperatures in 2011 that saw gas wellheads lock up and coal plants seize up.
Many power plants in the southern United States are not enclosed inside a building, with boilers and turbines exposed to the elements. This is by design. Leaving key power plant infrastructure outside prevents excessive heat build-up during warmer periods. But it can leave power plants vulnerable to cold weather, as a 2019 NERC report examining a 2018 cold snap in the southeastern U.S. makes clear.
Power planners in the Great Plains and Upper Midwest, by contrast, generally enclose their plants because they "expect to have to perform in cold weather conditions," said Susan Tierney, a former Department of Energy official who now works as a consultant. "That is not the case in Texas, so the unprecedented cold weather over so much of the state made it hard for the equipment to perform."
Power plant design is important within the context of the political debate about why the ERCOT grid has failed. Republicans have seized on wind turbines icing up as a warning that renewables are endangering the grid.
Democrats have responded by noting that gas and coal account for the majority of plant outages in Texas today. But both wind turbines and fossil fuels regularly perform at cold temperatures in northern climates, where they are designed to withstand winter's fury.
"Wind and solar were not significant contributors to what happened in Texas. They have planning around these particular events; they know in events like this wind and solar production will be low," Gilbert said. "That said, looking forward, wind and solar are going to have challenges with winter demand. And that is something we have to come to grips with as we try to decarbonize the electricity system."
Indeed, Texas' woes highlight some of the challenges facing American climate hawks. Renewable generation fades during the winter months in much of the U.S. as demand for energy surges. Summer, by contrast, is a relatively easier challenge because solar generation generally matches heat-induced demand spikes.
Solving that conundrum is exacerbated by the fact the country is seeking to make two energy transitions simultaneously, said Emily Grubert, a professor who studies energy systems at the Georgia Institute of Technology. One is a shift from dirtier power plants to cleaner ones. The other is a move to electrify buildings and cars that previously relied on oil and gas.
Expanding energy planning to include areas like building efficiency standards will be critical to help reduce demand and relieve the stress on the gird, Grubert said. Even so, she added, "it's going to be very, very difficult."