The nearly simultaneous shutdown of four gigantic power plants in Texas – capable of powering almost 1 million homes – drove the region to the brink of yet another energy crisis this week as temperatures soared, according to data analyzed by Bloomberg.

All told, generators across the second-largest U.S. state were either down for repairs or running at reduced capacity when triple-digit heat was baking Texas, according to data compiled by Wood Mackenzie Ltd.’s Genscape unit. But the biggest impacts came from just a handful of facilities: a Vistra Corp.-owned 1.15-megawatt nuclear reactor, and three other giant facilities owned by Talen Energy Corp. and NRG Energy Inc.

“We can’t really afford to lose a gig of capacity heading into an extreme weather event,” said Rebecca Miller, a Wood Mackenzie analyst, using industry shorthand for “gigawatt.”

For days, officials were pleading with homeowners to turn off appliances and ease up on A/C use, and closely watching supply gauges to avoid a repeat of February’s disastrous blackouts. At the peak of the crisis, electricity equivalent to the normal use of almost 2.5 million households was missing from the grid – just when it was needed most.

On June 7, almost a full week before the heat wave, a reactor at the Comanche Peak nuclear plant outside of Dallas dropped offline because of a transformer fire. Although the flames were quickly doused, the reactor is still down 11 days later and owner Vistra Corp. hasn’t said when it may return.

That wasn’t the only existing hole in the state’s generation capacity as temperatures began to climb. Talen’s Barney Davis natural gas-fired facility in the refining and chemical hub of Corpus Christi was transmitting just a fraction of its 933-megawatt capacity in the days leading up to the heatwave.

After demand began to soar on June 13, Barney Davis ramped up power production but the surge was short-lived and in a matter of hours, its output had collapsed. Talen has not disclosed what transpired and a representative declined to comment for this story.

NRG’s Limestone facility also was operating below its 1.66-gigawatt capacity going into the heatwave, and output plummeted on June 14 and again on June 15. The company’s 1.12-gigawatt W.A. Parish gas plant, meanwhile, was churning out electricity at a high rate going into the weekend before plunging, partially rebounding and then falling again, Wood Mackenzie’s data showed.

An NRG representative said the company is doing everything possible to ensure its units produce electricity but declined to comment on the outages.

At the height of the crisis, about 12.2 gigawatts of generating capacity was missing from the system, according to the stae’s primary grid manager. That included about 3 gigawatts of wind and at least 8 gigawatts of thermal facilities — which rely on natural gas, coal or nuclear fission.

The four plants can supply as much a combined 4.86 gigawatts, and their poor performance was probably the most significant absence during the heatwave, according to Wood Mackenzie.

“We have a constraint in supply, a spike in demand, things are on edge,” said Michael Webber, an energy expert and engineering professor at the University of Texas at Austin. “There’s so much thermal production offline right now that’s it’s creating problems. That’s the key.”

The performance of different types of generation has become a political flashpoint in the Lone Star state, with conservatives praising thermal power and bashing renewables while liberals do the opposite.

“If you ask a Republican if wind underperformed, of course they would say wind underperformed,” said Katie Bays, an analyst at FiscalNote Markets, adding that wind farms generally don’t produce as much power in hot weather. “Compared to my expectations, wind performed as expected.”