President Trump’s tariffs on imported solar panels mark the biggest blow he’s dealt to the renewable energy industry yet.

The U.S. will impose duties of as much as 30 percent on solar equipment made abroad, a move that threatens to handicap a $28 billion industry that relies on parts made abroad for 80 percent of its supply. Just the mere threat of tariffs has shaken solar developers in recent months, with some hoarding panels and others stalling projects in anticipation of higher costs. The Solar Energy Industries Association has projected tens of thousands of job losses in a sector that employed 260,000.

California could be hit particularly hard.

No other state generates as much solar electricity — both from rooftop installations and large-scale power plants — as California, or boasts as many workers. It accounts for roughly 41 percent of all solar electricity produced nationwide, while solar companies — many based here — employ more than 100,000 Californians, according to the Solar Energy Industries Association lobbying group.

Though some of California’s solar companies, such as Tesla, make panels in the United States, more focus on installing equipment made by others. A 30 percent tariff could strike a serious blow to their businesses.

The tariffs are just the latest action Trump has taken that undermine the economics of renewable energy. The administration has already decided to pull the U.S. out of the international Paris climate agreement, rolled back Obama-era regulations on power plant emissions and passed sweeping tax reforms that constrained financing for solar and wind. The import taxes, however, will prove to be the most targeted strike on the industry yet.

“Developers may have to walk away from their projects,” said Hugh Bromley, an analyst at Bloomberg New Energy Finance. “Some rooftop solar companies may have to pull out” of some states.

U.S. panel maker First Solar jumped 9 percent to $75.20 in after-hours trading. The Arizona manufacturer stands to gain as costs for foreign panels rise. The Solar Energy Industries Association estimated that the decision will eliminate about 23,000 U.S. jobs this year and delay or kill billions of dollars of solar investments.

Trump approved four years of tariffs that start at 30 percent in the first year and gradually drop to 15 percent. The first 2.5 gigawatts of imported solar cells are exempt for each year, the president said in an email.

The duties are lower than the 35 percent rate the U.S. International Trade Commission recommended in October after finding that imported panels were harming American manufacturers. The idea behind the tariffs is to raise the costs of cheap imports, particularly from Asia, helping those who manufacture the parts domestically.

American solar installers including SunPower, Vivint Solar and Sunrun jumped in after-hours trading.

MBA BY THE BAY: See how an MBA could change your life with SFGATE's interactive directory of Bay Area programs.

“A 30 percent tariff in Year One is bad,” said Gordon Johnson, a New York analyst at the Vertical Group, but “it’s less than what the consensus was.”

Jigar Shah, co-founder of investor Generate Capital and an outspoken advocate for the solar industry, went as far as to describe the decision as “good news.” The tariffs are “exactly what the solar industry asked for behind closed doors” to prevent a negative impact on companies, he said.

Sunrun said that while the decision lifts “a cloud of uncertainty,” it still runs counter to “consumers, bipartisan elected officials, many military personnel, and the 99 percent of American solar workers whom this tariff will harm in the coming years.” The San Francisco company called for the administration to clarify which countries won’t be subject to the tariff. Trump didn’t identify exemptions in his statement.

Rooftop solar installer Sunnova Energy Corp. said the tariffs will not deter the industry.

“The solar industry has been tested before and we have always shown our resiliency,” CEO John Berger said in a statement. Regardless of the tariffs, solar installer Tesla said it’s “committed to expanding its domestic manufacturing,” citing the “gigafactory” it opened in Buffalo, N.Y.

The duties won’t be entirely devastating for the U.S. solar industry, Bromley said, estimating that they’ll increase costs for large solar farms by less than 10 percent. The costs of a residential system, he said, will rise by about 3 percent.

The decision will “destruct some demand for new projects in the next two years,” Bromley said. “But they will likely prove insufficient in magnitude and duration to attract many new factories.”

For Trump, they may represent a step toward making good on a campaign promise to get tough on the country that produces the most panels: China. Trump’s trade issues took a backseat in 2017 while the White House focused on tax reform, but trade is now coming back into the fore: The solar dispute is among several potential decisions that also involve washing machines, electronics and steel.

Trump’s solar decision comes almost nine months after Suniva, a bankrupt U.S. module manufacturer with a Chinese majority owner, sought import duties on solar cells and panels. It asserted that it had suffered “serious injury” from a flood of cheap panels produced in Asia. A month later, the U.S. unit of German manufacturer SolarWorld AG signed on as a co-petitioner, adding heft to Suniva’s cause.

Clark Packard, a trade policy expert at the R Street Institute in Washington, described Trump’s decision as “regrettable,” warning that more jobs will be jeopardized by the tariffs “than could possibly be saved by bailing out the bankrupt companies that petitioned for protection.”

San Francisco Chronicle staff contributed to this report.

Brian Eckhouse, Ari Natter and Christopher Martin are Bloomberg writers. Email: beckhouse@bloomberg.net, anatter5@bloomberg.net, cmartin11@bloomberg.net