Texas power prices for Sunday surged more than 800% as searing heat pushes demand toward record levels and strains supplies on the state grid.

Electricity prices for the grid rose to more than $2,500 a megawatt-hour for Sunday evening, up from Saturday’s high of about $275, according to data from the Electric Reliability Council of Texas, the grid operator. The surplus of available power capacity on the grid versus power consumption will narrow to 1.6 gigawatts in the hour ending at 6 p.m. Sunday, a level that can trigger emergency responses, though Ercot has additional reserves it can tap to meet demand.

Read more: Texas Grid Keeps Power on With Aid of Alerts When Demand Soars

Ercot has issued a weather watch for Sunday and Monday “due to forecast higher temperatures, higher demand, and potential lower reserves,” the grid operator said in an emailed statement Saturday afternoon. “Ercot will continue to monitor conditions closely.”

Most of the state is under a heat advisory with large swaths of it under an excessive heat warning, according to the National Weather Service. Dallas may see temperatures rise to 107F (42C) on Sunday afternoon with a heat index of 111 for the second consecutive day.

While Texans continued about their daily activities Saturday, state officials warned residents to take precautions as they sought to mitigate the risk of extreme heat. Dallas is activating temporary cooling centers on Sunday, while in Fort Worth, authorities have cut back on the city’s twice-daily cattle herd.

Power usage on Sunday is expected to peak at nearly 84.4 gigawatts at about 4 p.m., which would be an all-time high in Ercot. But it’s not until later when the solar generation starts to wane at sunset that supplies become tighter.

--With assistance from Julie Fine.

Author: Naureen S. Malik