New England’s grid operator said Wednesday it declared a low-level emergency the previous evening as some power plants failed to generate electricity in the year’s first heat wave, potentially leaving the region short of electricity in reserve that could be tapped in an emergency.

ISO New England declared a “Power Caution” at 5:40 p.m. Tuesday, spokeswoman Mary Cate Colapietro said. It remained in effect until 10 p.m. when electricity demand fell. A Power Caution is an advisory that available electricity is insufficient to meet anticipated demand in addition to operating reserve requirements.

It is not a request that people limit their electricity use.

Widespread use of air conditioners typically creates a surge in electricity use during heat waves, pressuring power grids to meet demand.

Real time electricity prices briefly spiked Tuesday at nearly $1,900 a megawatt-hour, 10 times the typical price, signaling to markets a significant mismatch between supply and demand. It’s paid almost entirely by traders, not customers, said Dan Dolan, president of the New England Power Generators Association.

ISO issued the advisory following generator outages in the early evening and others that were unable to come online before peak demand, Colapietro said. The Power Caution authorized system operators to tap operating reserves to balance supply and demand, she said.

Peak demand for electricity is expected to rise as the heat wave persists. ISO forecast Wednesday morning it expects peak demand at 23,000 megawatts, up from 22,409 MW on Tuesday. The grid operator said conditions are “abnormal.”

It’s still far short of the all-time record demand in New England of 28,130 MW, which was set during a heat wave on Aug. 2, 2006.

Dolan would not identify which of the region’s power plants went offline or what caused the problem. Colapietro said ISO does not comment on specific power plants.

The Holyoke, Mass., grid operator serves 7.5 million retail electricity customers.

Bringing power from neighboring regions that also are sweltering and need electricity is not possible, adding to the problem. Just as utilities swap work crews to help restore power following storms, grid operators typically bring power from neighboring regions in extreme weather to supplement what’s available. But because a broad “heat dome” covered the Northeast, other areas are unable to help New England, Dolan said.

“It makes it harder to assume that others are going to ride to the rescue,” he said.

But a broader problem, Dolan said, is that falling electricity imports from neighboring regions, particularly Quebec, show that “truly dependable imported electricity has become more and more scarce.”

However, Colapietro cited ISO’s seven-day forecast for electricity capacity that includes daily imports of between 2,920 MW and 3,120 MW.

Dolan said ISO assumes when power is purchased at auction that about 2,000 MW will be made available from neighboring regions, primarily Hydro Quebec, in mutual assistance programs. However, unlike commitments made by generators, the assistance programs “have no requirements for performance or obligations. They are truly just assumptions,” he said.

“And in recent years those assumptions have been wrong consistently in key periods,” Dolan said.

He cited the destructive Christmas Eve storm in 2022 when a loss of 1,100 MW from Hydro Quebec from what was anticipated to what was delivered coincided with a reserve shortfall for 90 minutes. Power plant operators were hit with about $39 million in penalties.

Similarly, on the Feb. 3-4, 2023, “Arctic Weekend” when extremely low temperatures and wind chills set records in New England, the region sent more than 1,000 MW to Quebec, the New England Power Generators Association said. But at the peak on Feb. 3, “net exports from Quebec to New England were collapsing.”

In the most recent heat wave, Dolan said the “vast majority” of the electricity market is doing well. “This was a heat wave we’ve seen coming for a little while.”

“The scare yesterday afternoon redoubled a lot of efforts to make sure everything is online,” he said.

Peak electricity demand is expected to drop Friday and into the weekend as temperatures fall.