Getting backup power for all communications services is "infeasible" or "impossible" during extended power outages, according to filings that providers made with California regulators.

The reports from phone and internet companies, made available Tuesday, help explain why hundreds of thousands of people lost critical communications tools during Pacific Gas & Electric Co.'s October blackouts.

The California Public Utilities Commission has demanded answers about what failed, why, and how to prevent a repeat, and commissioners were set to grill executives from eight companies about the issue during a hearing Wednesday — just as another PG&E outage was to get under way.

Companies said most sites have batteries or generators, and some even have both, but they could last anywhere from two hours to a week. And many said it was logistically impossible or unsafe to deploy or refuel gas generators.

AT&T, which saw 3% of its network in the state affected, apologized to its customers. The company brought in hundreds of additional generators to grow a fleet of more than 2,000, "but even those were not enough to cover all our facilities that were left without commercial power," the company's filing said.

AT&T said "the unprecedented scope of the power outages is causing us to seriously reexamine our fixed and portable generator deployment capabilities for both our wireless and wireline facilities."

"Our network was not designed with the anticipation that a power company would shut off power to over 2 million people at once," AT&T said. "The unprecedented scope of the power outages is causing us to seriously reexamine our fixed and portable generator deployment capabilities for both our wireless and wireline facilities."

AT&T said it was working on an investment plan that would outfit all new cell sites with a permanent generator and retrofit existing sites where feasible.

AT&T's central offices and data centers have permanent generators that can run at least 72 hours. Its tens of thousands of smaller remote terminals that connect to customers have eight hour batteries.

In the second-highest areas of fire risk, eight of AT&T's 735 sites didn't have permanent generators or space to install a portable one. In the highest fire risk areas, generators couldn't be installed at nine of the 427 sites.

Verizon, which also had 3% of its service affected, said 57% of cell sites were out of service in Marin on Oct. 28. Across the state, only 8% of Verizon's sites were down, though that translated to more than 400 towers.

Verizon said a "substantial majority" of its sites have four to eight hours of battery backup while a "considerable majority" have generators that can run 24 to 72 hours. A few exceptions can't be served with portable generators "due to topographical limitations and/or technological challenges," Verizon said in a letter to an Oakland official. The company said it was still working on compiling the requested list of cell sites in high fire risk areas and their backup power.

T-Mobile said "essentially all" of its cell sites have backup power and the only ones that don't are in locations where local regulations or physical limitations restrict installation. Unlike others, T-Mobile did not disclose how long that backup power lasts.

During outages, the company deployed generators, which last between 48 and 72 hours without refueling. The company said it plans to acquire more portable generators in 2020.

Frontier, Comcast, Charter, and Cox, which offer broadband internet and wired phone service, said their central offices — which house important network equipment — have permanent backup power. But smaller parts of their network that connect to customers may not.

Frontier's central offices are powered by batteries, which last eight hours, or permanent generators, that survive at least 72 hours on a single tank of fuel. But other field equipment, which supports 15% of its services, have batteries that last four to eight hours.

Frontier said its network "will not operate for prolonged periods without commercial utility power." The company said it's working to improve facilities along parts of Highway 395, and Highway 299 between Redding and the coast.

Comcast said it prioritized keeping power and service to critical locations. The company's central offices have backup power -- either batteries lasting four to 12 hours, generators for longer or both. Beyond those sites, a number of other elements of the company's network only have backup power for four to 24 hours. Comcast said it wasn't safe or feasible to deploy generators to all of them.

During outages, Comcast said, it couldn't safely deploy all its 400 mobile generators because of "extreme fire weather conditions." The company's filing cited federal prohibitions on using portable generators during red-flag conditions unless the area has no vegetation.

California's power and service outages have caused politicians to push for regulations, which experts say companies have opposed in the past. Tejas Narechania, director of UC Berkeley's Center for Law and Technology, said that after Hurricane Katrina, the Federal Communications Commission tried to require cell sites to have at least eight hours of backup power. The communications trade group CTIA challenged the rule in court, and the George W. Bush administration sent the rule back to the FCC for further review. The commission withdrew it.

"There are relatively scant requirements for cell-site backup power," Narechania said. "To be sure, some carriers have installed backup power at many cell sites. But they're not required to do so, and we have little reliable information about how widely they've installed such backup power systems or how long backup power lasts."

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