US President Donald Trump on Saturday (Oct 10) signed a proclamation underscoring his support for revoking an exclusion from tariffs on some imported double-sided solar panels, and for raising the planned tariff rate to 18 per cent for 2021 from 15 per cent.

Trump said the domestic US industry was starting to increase production and market share of certain solar modules after he imposed tariffs on imports in January 2018, but further steps were needed.

Bifacial panels should not be excluded from the tariffs, Trump said, adding that doing so had limited the overall measures and would likely continue to impair their effectiveness.

"In light of the increased imports of competing products such exclusion entails ... it is necessary to revoke (the) exclusion and to apply the safeguard tariff to bifacial panels," Trump said in a proclamation released by the White House.

"To achieve the full remedial effect envisaged for that action, it is necessary to adjust the duty rate of the safeguard tariff for the fourth year of the safeguard measure to 18 per cent."

Solar farm developers, including Chicago-based Invenergy Renewables, had sued to maintain the exemption initially granted by the Trump administration, but it was later rescinded after officials realised it led to a spike in imports.

The United States in January 2018 imposed duties on solar panel imports beginning at 30 per cent and expected to drop to 15 per cent by 2021. Trump's announcement would put the rate at 18 per cent next year.

China and other producers dominate the bifacial technology market, a small but growing part of the solar panel market that costs more but produces greater power than traditional panels.

Consumers and importers have argued that higher tariffs will boost their costs and are unnecessary because domestic producers do not make the panels and face no harm from imports.

Domestic producers argue that solar farm developers could use either monofacial or bifacial panels, and higher tariffs would safeguard domestic production.