Electrical grid towers are seen on Wednesday in Pasedena as the heat wave begins. The strains on the power grid are expected to intensify as the heat peaks Monday and Tuesday.
Starting Monday, California will see its highest chance of rolling blackouts in the ongoing heat wave, grid officials said Sunday.
"This multi-day event is going to get much more intense," Elliot Mainzer, the CEO of the California Independent System Operator, which manages the state grid, told reporters on Sunday. California is facing, he said, "the highest likelihood of rotating outages we've seen so far" beginning Monday, he said.
The grid operator has asked Californians to reduce their electricity use between the hours of 4 and 9 p.m. on Sunday - a request known as a Flex Alert - and similar alerts are a near certainty on future days as the heatwave continues. The late afternoon and evening are when air conditioning demand is extremely high, and therefore the grid is particularly strained.
Demand is expected to peak at 45,000 megawatts on Sunday but then rise sharply on Labor Day as the worst of the heat arrives. On Monday, it is projected to rise to 48,967 MW, and to 50,099 MW on Tuesday - which would be close to an all-time high. On Wednesday, demand is expected to reach at 49,268 MW and remain high through most of next week.
To put that in perspective, California has exceeded 50,000 MW of demand only twice in the last 20 years. The all-time record was 50,270 on July 24, 2006. California's demand reached 50,116 one day in 2017.
Officials said that the grid is expected to go through various states of emergency on Monday in an effort to stave off rolling blackouts - and a key goal goal is keeping demand lower than forecast.
State officials plan to use all possible means of reducing demand and boosting supply in the late afternoons and evenings. Beside the Flex Alerts, options during the states of emergency include allowing businesses to use backup generators - a practice normally discouraged for environmental reasons - and encouraging other ways to cut demand, for example by reducing pumping at the energy-intensive State Water Project and having ships use on-board power rather than plugging in at ports.
One additional challenge is that in extreme heat, some plants that use water for cooling - like natural gas plants, especially older ones that are called into service for emergencies - cannot run at their full capacity and / or break down. Three gas plants in California have already experienced partial outages in the heat wave, according to Mark Rothleder, the chief operating officer of the grid operator.
"When you get these high temperatures, equipment does fail," Rothleder said. The length of the current heatwave also increases the challenge.
Grid officials are concerned that wildfires could shut down transmission lines, but so far that has not happened in a major way.
Hydropower imports from the Pacific Northwest have been "strong," Rothleder said, though he's hoping that California can import more power than is currently forecast for Monday.
In a positive sign, electricity demand on Saturday was roughly 2% lower than grid officials expected.
"That was encouraging that some of the steps that consumers are taking across California are contributing to the load reductions," Mainzer said, adding that he is very appreciative of residents' efforts.
Kate Galbraith is the San Francisco Chronicle's climate editor. Email: kgalbraith@sfchronicle.com Twitter: @kategalbraith