Power shut-offs like the one that darkened parts of California this week are expected to be a regular occurrence in the state for years during wildfire season as PG&E Corp. PCG 0.33% upgrades its aging equipment and completes overdue repairs. California utilities in recent years have resorted to so-called public safety power shut-offs in which they cut off electricity to certain areas to reduce the risk of their power lines sparking wildfires when wind speeds pick up.

PG&E, which serves 16 million people throughout Northern and Central California, cut power late Monday and early Tuesday to roughly 172,000 customers in 22 counties stretching from wine country to the Sierra foothills. The exact number of people affected is uncertain, but it likely tops 500,000 people, given census data on people per California household. PG&E said it expects to restore power Wednesday.

The San Francisco-based utility has been working to limit the scope of such shut-offs after they resulted in millions losing power last year, drawing the ire of California Gov. Gavin Newsom and state regulators. The most recent shut-off is less than a quarter of the size of PG&E’s largest one last October.

PG&E last year relied heavily on pre-emptive shut-offs after its equipment sparked a series of deadly wildfires in 2017 and 2018 that collectively killed more than 100 people and destroyed roughly 15,700 homes.

The company sought chapter 11 bankruptcy protection last year, citing billions of dollars in fire-related liability costs. It emerged from bankruptcy this summer after reaching settlement deals with fire victims, insurers and other claimants for more than $25 billion.

Last October, it cut power to more than two million Californians across 34 counties, some for days at a time, to reduce the risk of starting additional fires. It is the only U.S. utility to have ever initiated a weather-related shut-off of such size and duration.

After that event, then PG&E Chief Executive Bill Johnson said the company might have to rely on pre-emptive shut-offs for as long as a decade as it worked to make its electric grid safer. He said he expected them to become smaller over time.

Mr. Newsom roundly criticized PG&E for its handling of the shut-offs last year. His office didn’t respond to a request for comment following the latest blackouts Tuesday.