The solar industry was held up as a job-creation engine just a few years ago, when the fastest-growing job in America was that of the solar-panel installer. Solar sector employment quickly became key to the U.S. energy industry. Nationwide, about as many people work in solar companies as in natural gas, while coal mining employs less than half that amount, according to the Energy Futures Initiative.

But that hiring spree has cooled notably. Since 2016, the industry has shrunk for two years in a row, losing about 18,000 jobs, according to a yearly report from the Solar Foundation. Solar employment last year was 242,300, nearly 8,000 lower than in 2017 and down from a peak of 260,000 than in 2016.

One reason for the reversal: The Trump administration imposed tariffs on Chinese-imported solar photovoltaic units in January of 2018, which had the effect of delaying or canceling about $8 billion worth of utility-scale projects.

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"We have the trade policies and the tariffs that have been imposed on our industry, and they've had a detrimental impact on our ability to finalize projects and hire workers," Abigail Ross Hopper, president of the Solar Energy Industries Association, told reporters this week.

Large solar farms can take as long as 18 months to develop and, because of their size, they can be sensitive to shifts in component costs.

"There was a lot of uncertainty prior to the tariffs being announced in January 2018, and that uncertainty caused the larger projects to be delayed," Ed Gilliland, senior director at the Solar Foundation, told CBS MoneyWatch. "Once they found out what the tariffs were, it maybe wasn't as bad as we thought it would be."

A few months later, the Chinese government cut subsidies for solar production, which had the effect of dropping prices for solar modules. "And so now, despite the tariffs, the prices are now lower than they were last year," Gilliland added.

The U.S. imports about 90 percent of solar modules installed here, but less than one-tenth of them come from China.

Solar installers make up a large majority of the jobs in the industry, so a slowdown in the pace of solar installations can result in job losses, as happened in California, the country's largest solar producer. Solar jobs in the state dropped by more than 9,000, to about 76,000 jobs.

"Solar jobs are like construction-type activity — they're dependent on new solar being installed," said Andrea Luecke, executive director of the Solar Foundation. "So when California slows down the rate of installation, there will be fewer jobs."

But 29 other states bulked up their solar workforce. Florida added 1,700 jobs after the state permitted solar leasing last summer. These programs allow residents to rent a solar system, avoiding the upfront costs of installing panels on their home.

Illinois, which is on course to introduce a 100 percent renewable energy target this month, added 1,300 jobs. Alabama, Arkansas, Kansas, Wyoming and Montana saw solar job growth over 20 percent.

The different effects highlight the importance of state-level policies, said Luecke, who expects jobs to rebound slightly in 2019. "There's a huge backlog of projects right now that are in the queue, under development or in the planning phase," she said. "Really it's an unprecedented amount of generation that's in the pipeline."