But the program is an imperfect proxy for medically vulnerable communities, as Commissioner Liane M. Randolph noted.

Public health officers and consumer advocates say that the program under-represents who’s at risk in a shutoff area. For instance, people who live in buildings that receive just one electric bill — or in mobile home communities — can’t register.

And while the medical baseline program itself dates back decades, it’s only this year that PG&E began to use it as a warning mechanism in this kind of emergency. At the same time, a spokesman for San Diego Gas & Electric, the utility that pioneered public safety shutoffs after the 2007 Witch Fire, pointed out that regulations make eligible people who declare that they need air conditioning to be comfortable for a variety of medical conditions.

It's not yet clear how well PG&E warned medical baseline customers about the most recent shutoff event. According to earlier filings with the CPUC, PG&E warns its medical baseline customers via text, email or automated call.

If the customer doesn't confirm the warning, the utility will send someone out to knock on the door, said PG&E Senior Vice President Laurie M. Giammona.

If nobody answers the door, the utility hangs a tag, and considers the outreach successful. Giammona said that PG&E workers left door tags for about 700 people last week.

Deborah Kaplan of Oakland, who uses an electric chair, bed and ventilator and requires the use of an elevator, explained that being notified about power shutoffs "isn't all that helpful."

"If this is going to be a reality that we're going to have to get used to, then people who rely on power for survival need to have backup," she said.

The CPUC’s head of safety and enforcement policy, Elizaveta Malashenko, told lawmakers in August that the medical baseline program wasn’t designed for emergency response.

Commissioner Martha Guzman Aceves also wondered why some communities got backup generators, when others didn’t.

"Was wealth a factor?" she asked PG&E's Singh about why Calistoga got a generator.

Singh said that was not the case.

PG&E Executives Detail Communication Breakdowns

PG&E's Giammona said that the utility had not anticipated the number of people who would access the website.

She said that about 1.7 million users per hour were trying to use the PG&E website during the power shut offs. Normally, only about 7,000 users access the site per hour on a regular day.

CPUC President Batjer, who was present for some of PG&E’s decision making during the shutoff, said that employees too junior to make important decisions about technology were the ones tasked with system testing.

She said that she was "astonished at the lack of infrastructure" for a company of PG&E's size.

Batjer noted that dozens of local governments complained that they could not reach PG&E during the shutoff.

PG&E executives described calls with county and tribal governments where more than 1,000 people from local and tribal governments were on the line.

"Today a parade of PG&E executives told us what we already know — their October [power shutoff] was a massive failure in execution," state Sen. Jerry Hill told KQED via email.

Hill has been a consistent critic of PG&E since 2010, when one of the utility's natural gas transmission lines exploded in San Bruno, killing eight people in his district.

"It’s good to know they are attempting to belatedly rebuild their support services to meet customer needs ... But it would have been better for PG&E leaders to detail exactly why it’s going to take them 10 years to build the resilient electrical system we should already have, so that power shutdowns are truly a rare, last resort."

The California Senate opened its own investigation into how PG&E handled the shutoff on Thursday.

Molly Peterson contributed to this report.