(Bloomberg) -- The bill may soon come due for millions of Southeast Asian solar panels imported duty-free into the US that haven’t been installed by a Dec. 3 deadline.

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With the cutoff’s arrival Tuesday, companies that imported those panels between mid November 2022 and June 6, 2024 and haven’t deployed them are on the hook for tariffs ranging from 30% to more than 230%. The US Customs and Border Protection has vowed to “vigorously enforce” the requirement meant to deter stockpiling of duty-free panels, raising the risk of audits, inspections and potentially billions of dollars in retroactive tariffs for importers.

“That bill will shock a lot of people,” said Tom Beline, a trade attorney and partner in Cassidy Levy Kent’s Washington office. Robust enforcement is “tremendously important” to ensure the two-year tariff moratorium doesn’t mean we’re “living with a stockpile of panels that would never be installed.”

The crackdown heightens uncertainty for solar developers and manufacturers, coming on top of existing trade probes, questions about the longevity of tax credits for renewable power projects and President-elect Donald Trump’s vow to hike tariffs on a wide range of goods and suppliers.

At issue is a tariff holiday President Joe Biden ordered in 2022 to blunt the impacts of a US government trade probe that spooked renewable developers and chilled solar installations nationwide. Ultimately, the Commerce Department concluded manufacturers were evading longstanding tariffs on Chinese solar imports by assembling the equipment in Cambodia, Malaysia, Thailand and Vietnam.

Although the government extended those duties to solar gear from the Asian nations, Biden’s order effectively meant tariffs didn’t apply until early June. To counteract a surge in duty-free solar imports from the region, the administration set the December deadline for using or installing the equipment.

Federal regulators have spent months warning importers they’ll have to prove modules have been “utilized” or pay duties retroactively. And they’ve taken pains to explain what qualifies, making clear that creative measures — from destroying affected panels to temporarily installing them at warehouses — aren’t enough to dodge the duties.

Companies raced to deploy the imported equipment as Tuesday’s deadline neared. But analysts estimate some 30 to 40 gigawatts of affected imports are still waiting to be used — at least two-thirds of current annual US panel needs. Based on import and installation data analyzed by BloombergNEF, more than 30 gigawatts of modules in the US missed the deadline.