The blackouts affect half a million people – or nearly 180,000 customers – in 15 counties in Northern California.

Dangerously windy weather sweeping through the state brought power outages to Northern California as the state’s largest utility staged blackouts designed to prevent catastrophic wildfires.

Pacific Gas & Electric Co (PG&E) on Wednesday began rolling blackouts stretching from the Sierra foothills in the northeast to portions of the San Francisco Bay Area. A couple of counties kept their power until after midnight.

The blackouts affect half a million people – or nearly 180,000 customers – in 15 counties, and PG&E warned that a second round of outages could occur over the weekend when winds return to the region.

Meanwhile, a wind-driven wildfire has forced evacuations north of San Francisco in Sonoma County. The blaze near Geyserville had grown to 40.5 square kilometres (15 square miles) by early Thursday, according to the state firefighting agency Cal Fire.

In the south, where hot, dry Santa Ana winds were expected to hit Thursday, Southern California Edison warned that it might black out about 308,000 customers – perhaps 750,000 people – depending on the forecast.

San Diego Gas & Electric warned of power shutoffs to about 24,000 customers.

The utilities have said the precautionary blackouts are designed to keep winds that could gust to 97km/h (60mph) or more from knocking branches into power lines or toppling them, sparking wildfires.

Electrical equipment was blamed for setting several fires in recent years that killed scores of people and burned thousands of homes.

“We understand the hardship caused by these shutoffs,” PG&E CEO Bill Johnson said Wednesday. “But we also understand the heartbreak and devastation caused by catastrophic wildfires.”

The latest outage comes two weeks after PG&E shut down the power for several days to about two million people in northern and central California.

More blackouts possible

The current outages will last about 48 hours, the utility said. But its seven-day forecast shows a likelihood of another planned blackout across a much larger area. The timing was not clear but it could start as early as Saturday, when even heavier winds are expected to move through.

“This could be the strongest wind event of the season, unfortunately,” PG&E meteorologist Scott Strenfel said.

Strenfel called the current wind event a “California-wide phenomenon.”

The small city of Calistoga, in the Napa Valley, known for its hot springs and wineries, was among those hit by Wednesday’s outage.

“It’s very frustrating,” said Michael Dunsford, owner of the 18-room Calistoga Inn, which has rented two powerful generators for the month at a cost of $5,000. Like many, he felt the outages need to be better managed, better targeted and less expansive.

“Right now, we have no wind. Zero. I don’t even see a single leaf blowing. Did they really have to cut the power right now?” he said, shortly after the lights went out Wednesday afternoon and he revved up the generators. “When the wind picks up to 40 mph maybe that’s a good time to close the power.”

“They’re not appreciating enough the impact this has on everybody,” he said about PG&E.

Some of the frustration was being taken out on PG&E employees, the company’s CEO said.

Johnson said Wednesday that a PG&E employee was the target of what appeared to be a deliberate attack in Glenn County. He said a projectile that may have come from a pellet gun hit the employee’s front window. The employee was not hurt.

“There is no justification for this sort of violence,” Johnson said. “Wherever you see crews they are there to help you.”

Mandatory evacuations were prompted east of Geyserville after a wildfire sparked in northeast Sonoma County along the Lake County line late Wednesday.

The Press Democrat reports that according to dispatch reports, the Kincade fire spread to about 4 square kilometres (1.5 square miles) by 11pm.

Cal Fire spokesman Will Powers said the blaze near the Geysers area was burning at a “dangerous rate.”

Sonoma County Supervisor James Gore said PG&E was better this time about getting information to people who would be affected, but he was still astonished by the need to resort to large-scale blackouts.

“I am a big believer in shutdowns to prevent fires. But the thing that erodes public trust is when it doesn’t make sense,” he said. “You say, ‘God, I know if we can put a man on the moon … we can manage a (power) grid.'”