The state’s power-grid managers were on high alert Sunday, as extreme temperatures threatened to push California’s energy demand past its supply, while winds forecast later in the week could cause more energy shutdowns.

For the second day in a row, as scorching temperatures reached record-breaking peaks, the California Independent System Operator was calling on Californians to conserve energy to protect the power grid as it faced a supply shortage. Officials said Sunday afternoon that they were expecting 2.3 million customers could lose power, starting by about 7 p.m.

“Sunday is probably sizing up to the most challenging day of the year,” California Independent System Operator Vice President Eric Schmitt said at a news conference. “We have very severe conditions in the system with highs and lows projected at about 49,000 megawatts on the system.

“Our highest load capacity is about 47,100 megawatts, so I think it’s fair to say that without really significant conservation and help from customers today, we will have some rolling outages.”

The Flex Alerts were set from 3 p.m. to 9 p.m. Sunday and Monday to reduce the strain on the power grid. Millions of people were expected to turn their thermostats down to escape the second substantial wave of heat this summer, pushing the state’s energy supply to the brink. If they are ordered, rolling blackouts are likeliest between 5 and 9 p.m.

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On Saturday, the system operator declared a Stage 2 emergency after about 1,600 megawatts went offline because of fires raging in Southern California. That capacity, enough electricity to power about 1.2 million homes, remained offline Sunday morning. Officials said that they gained back about half of the generation they lost Saturday afternoon by the end of the day.

A large part of the capacity that was lost also came from massive solar farms in the desert that connect to the Southern California grid, as well as power lines impacted by the Big Creek Fire in the Sierra Foothills. Though the power lines affected are hundreds of miles away from the Bay Area, Phipps says that constraints on solar and hydroelectric power going into Southern California could still impact customers here.

“Energy at one plant can feed homes on the complete other side of the state,” Phipps said. “If there’s energy lost anywhere, it could impact customers across California.”

Late Saturday, the Creek Fire in Madera County forced the closure of a 915-megawatt hydropower station, further constraining grid resources. John Phipps, the Cal ISO director of realtime operations, said Sunday that capacity could “change in an instant.”

It’s not the first time this summer that the Cal ISO has warned Californians of rolling blackouts; in August, a heat wave spawned four days of warnings, but conservation efforts staved off the outages. Regulators have also warned that Public Safety Power Shutoffs may happen later this week, as the weather forecast also notes high winds that, combined with the heat, could pose a serious threat to start new fires.

Power-grid officials are asking all Californians to conserve energy by shifting energy use to morning and nighttime hours, when temperatures remain high and solar production falls due to the sun setting. Air conditioners should be set to 78 degrees or higher; consumers should avoid using large appliances during the Flex Alerts, and keep devices unplugged when not in use.

On top of outages that could come Sunday, Californians should also be prepared for Public Safety Power Shutoffs this week, as utilities may choose to de-energize power lines in order to prevent wildfires amid heat and strong winds. Though Cal ISO officials are looking at fires closely to see what facilities are threatened, there’s still no way of knowing how a fire will behave.

But with so many fires raging across the state, Phipps said “it’s hard to reconfigure the network if you have multiple impacts on your transmission system.”