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The number of people without power across Texas rose to an estimated 4 million on Monday, with up to 300,000 people at a time affected in the San Antonio area.

As unprecedented winter cold, snow, and ice reached deep into Texas, power grid operators took emergency measures early Monday to keep the grid from failing uncontrollably. Shortly before 1:30 a.m., the Electric Reliability Council of Texas (ERCOT), the state’s grid operator, triggered rotating blackouts, requiring major transmission companies such as CPS Energy to reduce their share of the electricity load.

CPS Energy began rotating outages so customers would only face 15 minutes every hour without power, CPS Energy President and CEO Paula Gold Williams told reporters Monday afternoon. Later on, “that got inverted,” she said, with only 15 minutes on and 45 minutes off. Conditions grew “even tighter” in some areas, she said.

Gold-Williams said these rotating outages could extend at least until late Tuesday and up to “several more days.”

“Until we can get more [power generation] capacity on, the temperatures go up, all those things, it’s going to be a fairly inconsistent result,” Gold-Williams said.

In CPS Energy’s territory, the number of locations without power hovered around 200,000 for most of Monday, reaching 293,000 as of 7:30 p.m. Some people reported going all day without electricity.

“It’s been completely out since before 2 a.m.,” Beacon Hill resident Cassandra Dann said in a Facebook comment around 3 p.m. Monday. “Hasn’t come on at all. Many of my neighbors [are] in the same boat.”

Even San Antonio Mayor Ron Nirenberg said he was without reliable power.

“Every major Texas city is going without [power] tonight, and I share your frustration with the timing of these outages,” Nirenberg said in a Facebook post Monday evening. “ERCOT needs to answer for how millions were forced to weather this cold in this manner.”

CPS Energy officials say they rotated outages randomly across the utility’s service area, sparing only areas near critical sites such as hospitals, military bases, and medical research centers.

CPS Energy has more than 250 circuits eligible to be shut off, Chief Customer Engagement Officer Rudy Garza said, with another 125 or so circuits not eligible for rotating shut-offs because they serve a critical site.

“If you’re in a certain critical circuit … you may not be seeing much of anything,” Gold-Williams said.

Meanwhile, Garza said the utility logged 5,000 to 15,000 customers with equipment-related outages at any given time on Monday. Those longer-lasting outages came from downed lines, fallen poles, and other equipment problems.

The differences in experiences led to some neighborhoods with longer and more frequent outages than others located within only a few miles.

“Near downtown and without power since just before 2 a.m.,” Alissa Howard, a local graphic designer, wrote in a Twitter comment Monday. The air was cold enough indoors to see her breath inside, she said.

“Debating a third pair of socks,” Howard said. “Using last cell power bank and can’t figure out what’s going on. It’s been a wild time.”

But Fernando Ortega, a resident of the Lavaca neighborhood, also near downtown, said in a Facebook message that his power would come on for five minutes or so once every 30 to 50 minutes.

Southside resident Patricia Zapata told the San Antonio Report on Twitter that her family was coping with five minutes of electricity every 15 to 40 minutes.

“We are sitting in my car so we can charge our phones and get warm,” she said.

Barbara Powell, who lives in the Northside neighborhood of Deerfield, said she and her husband decided to stash all the food from their refrigerator outside on their deck around 6 a.m. after four hours without electricity.

“Right now, my husband, daughter, and I are bundled up together in bed with our dog,” Powell said. “Wish we had two more dogs. This is definitely a three-dog day.”

The situation began worsening on Sunday, with ERCOT calling for voluntary conservation measures into the evening. The amount of unspoken-for supply on the grid began to shrink beginning around 11 p.m., when power generators across the state began tripping offline, said Dan Woodfin, ERCOT senior director of system operations.

He attributed these early shutoffs to “a significant number of natural gas fuel generators,” with many such generators seeing natural gas in tight supply for both power and heating fuel. Meanwhile, “we had some wind turbines that were out due to icing buildup,” he said.

Even as demand swelled Sunday to an all-time winter high of 69,222 megawatts, more power plants began to come offline. As of around 10:30 a.m. Monday, ERCOT had roughly 34,000 megawatts of generation capacity that was unavailable, Woodfin said. These power plants included natural gas, coal, and nuclear generators.

“Most of the plants that went offline during the evening and morning [Monday] were fueled by one of those sources,” Woodfin said early Monday, adding that “we don’t know exactly why they tripped offline yet.”

Monday marked the fourth time Texas’ grid has ever implemented such rotating outages, also called rolling blackouts. The most recent was Feb. 2, 2011, when a harsh cold front brought similar consequences for the power system.

In such events, ERCOT requires transmission companies such as CPS Energy to reduce their demand by the percentage of demand their customers take up on the Texas grid. In CPS Energy’s case, that’s about 7% of ERCOT’s total demand, Woodfin said.

“This event was well beyond the design parameters for a typical or even an extreme Texas winter that you would normally plan for,” Woodfin said. “The result is what we’re seeing today.”

The amount of demand ERCOT directed CPS Energy to shed ranged from 1,100 megawatts to 1,200 megawatts, roughly three times more demand than CPS Energy has ever been asked to cut since the ERCOT system began in the 1970s, Garza said.

Those who still have power can help their neighbors by conserving as much as possible – turning down thermostats a few degrees, unplugging unused appliances. These make a huge difference in reducing the length of others’ outages, CPS Energy officials said.

“When we’re making it bearable but not super warm, that means that kilowatt-hour of power can be shared somewhere else where it’s colder, and they can get it up to 68 degrees,” Gold-Williams said. “It could be the difference between the situation getting worse and the situation getting better faster.”

CPS Energy is a financial supporter of the San Antonio Report. For a full list of business members, click here.

Brendan Gibbons is a former senior reporter at the San Antonio Report. He is an environmental journalist for Oil & Gas Watch.