LOS ANGELES (CNS) — Executives with the California Independent System Operator -- which controls roughly 80 percent of the state's power grid through Southern California Edison, Pacific Gas and Electric Company and San Diego Gas & Electric -- said statewide rolling blackouts could affect millions of Californians Monday evening.

California ISO declared a statewide Stage 2 Emergency at 4:25 p.m., saying it anticipated asking utilities to shed 1,400 megawatts of power between 5 p.m. and 6 p.m. That dropped from an earlier estimate of a 4,400-megawatt shortfall due to consumers conserving power, lower temperatures and other measures taken by the agency, according to the release.

Nearly 1 million households could still be affected, based on estimates provided by ISO earlier Monday.

California ISO said outages were likely to last about two hours on average, while San Diego Gas & Electric Company said it might need to shut off power to more than 100,000 homes and told its customers to prepare to be without power for roughly an hour. Southern California Edison said the timing of outages and number of households impacted would depend on how much power ISO asked the utility to shed.

All California households were asked to take measures to conserve energy from 3-10 p.m. That request includes Los Angeles, even though the Los Angeles Department of Water & Power, which supplies the city's electricity, should not be affected by the outages, because it runs its own plants and has sufficient supply to help support the state grid.

"We own our own power plants and transmission lines and had enough supply to meet demand and required reserves," the utility said in a statement. "We have called on our customers to continue conserving to help the state grid and reduce strain on the system.

"LADWP is not a part of the CAISO, but is assisting the state grid by providing energy to them to help reduce the number of customers affected by rolling blackouts elsewhere in the state."

A Southern California Edison spokeswoman said the first outages, if required by the state agency, would occur within a pre-identified 80 "blocks" of users geographically dispersed throughout the utility's service area.

Those blocks are mapped across multiple geographies, spanning several cities and communities, so that no one area would lose all of its power. Fire stations, hospitals and other public safety facilities are excluded from the blocks.

ISO said it could take longer for utilities to bring back power in areas at high risk for fire, but SDG&E said those communities will be excluded from rotating outages.

More widespread rotating blackouts could occur in the days ahead and some customers who were already hit by outages this weekend could lose power again, according to SDG&E.

Southern California Edison customers can see whether their home or businesses is in the queue to lose power by checking online.

Earlier Monday, Gov. Gavin Newsom warned Californians to be prepared for more rolling blackouts over the next 72 hours -- which he called "very likely" -- as the state struggles to meet demand for electricity during a historic, record-breaking heat wave.

California ISO President and CEO Steve Berberich said several factors combined to create the supply/demand imbalance.

"We have a perfect storm going on here," Berberich told reporters in an afternoon briefing.

Newsom agreed that despite using "all the tools in the tool kit" to meet demand, "we are likely to fall short" as oppressive temperatures stress the state's energy system.

"Rather extraordinary weather conditions" also have put firefighters under enormous pressure as they battle wildfires across the state, Newsom said.

"These next few days we are anticipating a challenge," Newsom said, as he outlined efforts to buy more power from out of state, work with the biggest users of energy -- including large container ships --- to limit use, and better manage the state's supply.

Newsom said the state should have been better prepared to avoid unscheduled blackouts.

"You can't control the weather but you can prepare for weather events," Newsom said. "We failed to prepare ... I'm ultimately accountable."

Big power users are being allowed to shift to backup sources and stored energy that is typically restricted as state officials work to urgently deploy more resources systemwide, according to Newsom.

Newsom pointed to the state's shift to renewable resources as part of the reason for the supply shortage. Shutting down polluting gas power plants has created gaps in the state's energy supply, he said.

While the state remains committed to a greener future, Newsom said, "We cannot sacrifice reliability" and promised that officials would be "much more aggressive ... in making sure that is the case."

Gas power plants, sometimes referred to as "peaker plants" to be relied on only to meet peak demand, could conceivably be brought back on line within the 72-hour period, Berberich said, but maintenance issues could pose problems. California ISO would not control that process and Berberich could not say which power plants might be considered.

Newsom announced that he had signed an emergency proclamation to free up energy capacity.

That proclamation suspends permitting requirements related to air quality issues and allows some users and utilities to access backup energy sources to relieve pressure on the grid during peak times.

Availability of out-of-state supply, which accounts for about 25 percent of California's energy, is limited.

"We can't get the energy that we would normally get out of state because it's being used to serve load natively," Berberich told reporters. "On most days, it does close the gap."

Berberich said the state's Public Utilities Commission could have avoided the outages if it had moved more quickly to better secure imports and put enough resources in place to meet peak demand.

A statewide Flex Alert calling for residents to voluntarily conserve electricity remains in effect through Wednesday. Officials are also urging businesses statewide to restrict their usage. In some cases, the state is asking business owners to support outreach to their customers about conserving energy.

Newsom cited the electric vehicle manufacturer Tesla as one business working closely with the state.

The California Independent System Operator issued the Flex Alert on Sunday, stating there was insufficient energy to meet high consumer demand during the record-breaking heat wave.

To minimize the need for controlled outages, residents were asked to use air conditioning early in the day and set thermostats at 78 in the afternoon and evening hours, while avoiding the use of major appliances between the hours of 3 p.m. and 10 p.m.

The alert followed weekend blackouts that began Friday -- the state's first such outages since 2001 -- that Newsom said came without warning. He called the weekend service disruptions "unacceptable."

On Saturday, high temperatures increased electricity demand while one power plant was down and wind power fell short, prompting a Stage 3 Electrical Emergency that lasted 20 minutes. It was called at 6:28 p.m., making rolling outages imminent or in progress, according to the California Independent System Operator.

Over the weekend, state officials worked to bring more resources online, including increased power from LADWP, the California State Water Project and investor-owned utilities, according to the governor's office.

LADWP is expected to supply 900 megawatts of power to the state grid Tuesday.

"That's thousands and thousands of homes that would otherwise lose power," Berberich said.

On Sunday, Newsom met with members of the California Independent System Operator, California Public Utilities Commission, California Energy Commission, Governor's Office of Emergency Services and senior administration officials.

In a letter written after the meeting, Newsom said the blackouts were called Friday and Saturday without notice and demanded an investigation.

"Residents, communities and other governmental organizations did not receive sufficient warning that these de-energizations could occur. Collectively, energy regulators failed to anticipate this event and to take necessary actions to ensure reliable power to Californians," Newsom wrote. "This cannot stand. California residents and businesses deserve better from their government."

Power providers say a lack of supply from sources outside the state contributed to the shortage, as other Western states struggled to meet their own demand during the heat wave.

During his midday Monday news conference, the governor promised the investigation would be swift and comprehensive.

Residents struggling to stay cool in the past have been able to access theaters, swimming pools, public libraries and community centers, but these gathering places have been largely closed due to COVID-19. A list of cooling centers open across Los Angeles County is available at lacounty.gov/heat.

In order to save as much energy as possible, consumers are urged to shift their use to morning and nighttime hours and avoid using appliances and air conditioning in the late afternoon and evening hours. Late in the day, temperatures remain high but solar production falls as the sun sets.

Tips for Conserving Energy Include:

• Set air conditioning thermostats to 78 degrees
• Defer use of major appliances
• Turn off unnecessary lights
• Unplug unused electrical devices
• Close blinds and drapes
• Rely on fans when possible
• Limit time the refrigerator door is open

Consumers can also pre-cool their homes during earlier hours, when they can also charge electric vehicles, medical equipment, mobile devices and laptops and run major appliances. Pool pumps can be set to run in the early morning or late at night.

Customers in danger of losing power should turn off their lights -- leaving one on to alert them when power returns -- and unplug electronic devices, televisions and air conditioners in anticipation of outages to prevent damage.

Utilities advised keeping a fully charged cell phone on hand, along with flashlights and batteries. Leave refrigerators running with the door closed to keep food cold. Motorists should be alert for the possibility that traffic lights and signals may be shut down.

Customers who rely on electric or battery-dependent medical technologies, such as breathing machines, power wheelchairs/scooters, and home oxygen or dialysis, were urged to be prepared to activate their emergency plans.

More conservation tips, along with safety measures to be taken in the event of outages, can be found at flexalert.org.