No state uses as much solar power as California, or boasts as many solar jobs.

So President Trump’s decision Monday to slap a 30 percent tariff on imported solar panels could have an outsize effect on the Golden State.

California in the past decade lost many solar manufacturing startups — most notoriously, Solyndra — that were unable to compete against low-cost panels from Asia.

And yet those same inexpensive imports, their prices dropping year after year, fueled the growth of other companies that sell and install solar arrays. Photovoltaic cells even grew cheap enough to be used in large-scale power plants, an idea once considered wildly unrealistic.

Now the solar industry employs more than 100,000 Californians, about 38 percent of all solar workers nationwide. The state’s large- and small-scale solar installations, together, can generate more than 20 gigawatts of electricity at full production, roughly the same output as 20 nuclear reactors.

“California in particular, which has been leading the country in developing renewable resources, will certainly be impacted,” said Jerry Bloom, a senior counsel with the Winston & Strawn law firm, which specializes in energy transactions and project development for the electric power industry.

“The reality is we don’t know the exact impact,” he said. “But, in essence, you are making this product less attractive because of the costs.”

The tariffs were sought by SolarWorld Americas, an Oregon subsidiary of a German company, and Georgia’s Suniva, the majority of which is owned by a Chinese business. Complaining that cheap foreign imports had devastated U.S. solar manufacturers, the two businesses sought 50-percent tariffs, saying the move could add more than 114,000 jobs nationwide.

Most of the U.S. solar industry, however, saw the trade case as a threat. The Solar Energy Industries Association, an advocacy group, estimated tariffs could kill as many as 23,000 jobs this year and throttle solar power’s growth.

The tariffs announced Monday will phase out after four years, dropping from 30 percent in the first year to 25 percent in the second, 20 percent in the third and 15 percent in the fourth. And in each year, the tariffs will kick in only after American companies import enough panels to generate 2.5 gigawatts of electricity. In the third quarter of 2017, the U.S. solar market installed enough photovoltaic panels to generate just over 2 gigawatts, according to GTM Research.

Trump’s decision could benefit some domestic companies.

Palo Alto’s Tesla, which absorbed SolarCity last year, is revving up production of its own solar cells at a new factory near Buffalo, N.Y. The factory also is making Tesla’s integrated solar roof, first unveiled in 2016. In a brief statement Monday, Tesla said it was committed to increasing production “regardless of the solar tariff decision today.”

San Jose’s SunPower, in contrast, predicted that it could suffer more from Trump’s decision than any other company.

SunPower manufactures its own solar cells in the Philippines and Malaysia, packaging them into panels at facilities in Mexico and France. Last year, the company also announced a joint venture to manufacture panels in China and opened a research and development production line in San Jose.

“Unfortunately, SunPower — an American company with the most at stake in this case — will suffer collateral damage in a case that clearly was targeted at Chinese manufacturers,” the company’s CEO, Tom Werner, said in a statement.

Sunrun of San Francisco, which installs and operates solar arrays, said it hoped supportive state governments would look for ways to overcome the tariffs’ negative effects. California’s policies encouraging renewable power have been widely credited with nurturing the industry.

“Although lifting a cloud of uncertainty, the outcome of the trade case is contrary to the will of consumers, bipartisan elected officials, many military personnel, and the 99% of American solar workers whom this tariff will harm in the coming years,” Sunrun co-founder and Executive Chairman Ed Fenster said in a statement.

Although California leads the nation in solar jobs and generation, the state’s market for residential solar power weakened considerably last year.

According to GTM Research, residential solar installations in the first three quarters of 2017 dropped 23 percent compared with the same period of 2016. Analysts have blamed the drop on a retrenchment by some of the big national installation companies, which are trying to cut the amount they spend acquiring customers. In addition, analysts questioned whether many of the early adopters most interested in buying or leasing rooftop arrays had already done so.

Bernadette Del Chiaro, executive director of the California Solar Industries Association lobbying group, argues that the state’s residential solar market has plenty of room to grow. She blames the recent weakness on the transition to new rules for compensating homeowners with solar panels who send excess electricity to the grid, as well as an unusually wet rainy season, which delayed many installations.

“We’ve got a resilient and strong industry here, and a motivated customer base,” Del Chiaro said. “Any decision that blankets an entire industry across the country with increased costs is not good. That said, we’re pretty used to what we call the ‘solar coaster.’ We’ve been able to dust ourselves off before and find ways to cut costs.”

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF