When the lights went out this week, Susan Illich of Sebastopol didn’t just lose power. She also lost water.

That’s because, like thousands of residents in Sonoma County, she relies on a private well that operates with an electric pump.

“The one common thing everyone needs access to is water,” she said.

If the fire had gotten closer, she would have had no water to hose down her roof.

“Water puts out fire,” she said. “My basic rights to fend off fire that could have killed me and my pets and damaged my home was obstructed.”

The recent spate of PG&E blackouts to avert wildfires highlighted how exposed many residents are to losing the essential resource of water.

“From the Golden Gate Bridge to the Oregon border, tens of thousands of residents depend on domestic wells for water supply to be able to live their lives,” said state Sen. Mike McGuire, D-Healdsburg. “During these power shut-offs, they were all left vulnerable.”

Up to 2 million Californians are served by an estimated 250,000 to 600,000 private domestic wells, according to the State Water Board. Generally they’re in rural and unincorporated areas where it’s expensive to run pipes from municipal water systems.

Clare Pace, an environmental science researcher at UC Berkeley who is studying the topic, estimates that 183,000 people use wells in the nine-county Bay Area and 298,000 in the 15 California counties north of the Bay Area. People who rely on wells tend to be lower income and are more likely to be people of color and have lower English-language proficiency, making them less able to advocate for themselves, she said.

“Many people in cities don’t understand how dependent rural communities are on domestic wells,” said Peter Gleick, a climate and water scientist who is president emeritus of Oakland’s Pacific Institute, a think tank that studies water issues. “Power outages in rural areas sometimes completely cut off the water supply. Water is critical to us; much more fundamental than electricity.”

Farms also rely on wells that use pumps dependent on electricity.

“For the entire week, folks were needing to water their grapes, their livestock, their crops,” McGuire said. “This has taken an incredible toll on agricultural communities.”

In the Placer County town of Colfax, 65-year-old Ruth Boyd said she stockpiled about 60 gallons from her well before the two-day outage — and it was barely enough.

Her two black horses, Luna and Rena, each drink at least 10 gallons of water a day.

“I filled up every container I had lying around,” she said. “For me, It was only inconvenient. But for people with medical conditions, it would have been a real hardship.”

Sewage was also an issue. Lake County saw backup generators at sewage plants burn out from a four-day outage, McGuire said.

State officials had to scramble “to send additional generators to stop any potential flow of raw sewage into Clear Lake,” he said.

Carol Huchingson, Lake County administrative officer, said employees of the county water and sewage agency worked 16- or 18-hour days, constantly shuffling generators among sewage-pump stations. The county agency also had trouble pumping water, but residents heeded its pleas to limit usage so the taps never ran dry, she said.

In Sausalito, shoreline residents and houseboat dwellers need electricity to pump the contents of home-sewage tanks uphill to sewer pipes.

“For 65 hours, those of us in the houseboats couldn’t use the toilet or shower or use any water without the risk of flooding our homes with sewage,” said Gloria Packard. “If the tank overflows into the house, you can imagine the mess, as well as dire health consequences.”

The outages did not disrupt water for firefighters, said Scott McLean, a spokesman for Cal Fire. Domestic water wells don’t provide many gallons per minute anyway, although agricultural wells do.

Fire trucks carry gas-powered pumps to tap into local resources. “We can pull water from ponds, rivers, creeks, lakes, ditches, even swimming pools,” he said.

Sometimes people in remote areas invest in 2,500-gallon tanks set up for fire-hose coupling, said McLean, who saw that as a Butte County firefighter. “Rural areas don’t have fire hydrants,” he said.

Many people with well water have storage tanks, but big ones are pricey. Illich, for instance, had a 55-gallon trash can of water that sustained her and her dogs.

“You could have a water storage tank of a few thousand gallons to tide you over for a few days, but that is expensive, and foothill counties are poor,” said Maren Bell, who lives outside San Andreas in Calaveras County.

Bell and all her neighbors lost water when the power went out, and it’s likely that many of the county’s 45,000 residents were also cut off since only a few thousand are on municipal water, she said.

Other backup options are also expensive.

“We have been working on the installation of a generator, but it’s not a simple job if it needs to pump the well,” emailed Judith Woodard, who lives with her husband in a residential neighborhood near the Napa Valley Country Club in the city of Napa. They use a well, “so no power, no water,” she said.

No water means no indoor plumbing.

Ronit Rubinoff, executive director of Sonoma County Legal Aid, who lost power and access to well water in her Sebastopol home for four days, said it was “rough because you can’t flush your toilet, you can’t wash dishes, you can’t take a shower.”

“I can do without light and even without heat, but no water is a lot tougher,” she added. “We ended up siphoning water from a neighbor’s hot tub just for toilet flushing.” Other neighbors had filled up bathtubs with water to prepare.

While residents with wells scrambled to conserve water they’d stored, residents who had access to municipal water systems also needed to be mindful of turning on the tap during the shut-offs.

Many water districts warned customers to conserve water to ease the demand on backup generators.

“We had no changes to water supply, water quality or water pressure,” said Andrea Pook, a spokeswoman for the East Bay Municipal Utility District. The agency, which serves 1.4 million customers, spent $400,000 to rent generators and budgeted about $75,000 for fuel for the five-day shut-off, she said.

Even for people who prepare by filling bathtubs ahead of an outage, moving water with siphons and buckets requires physical strength and agility, Rubinoff said. That means it’s a challenge for seniors.

“If you’re an elder and don’t have water, you’re not able to stay in your house,” she said.

San Francisco Chronicle staff writer Steve Rubenstein contributed to this report.

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com Twitter: csaid