Shares in European clean energy groups plummeted on Wednesday after Donald Trump was elected president of the United States, as investors fretted over a potential dismantling of U.S. support for renewables and climate policy.

Trump had vowed to scrap offshore wind projects through an executive order on his first day in office and to roll back climate regulations implemented under President Joe Biden.

That includes leaving the Paris Agreement, under which countries pledge to limit global warming, and potentially undoing Biden's flagship Inflation Reduction Act, which provides massive subsidies and incentives to clean energy technologies.

An offshore wind project in the German North Sea, owned by EnBW and Enbridge is seen in this handout image taken August 2021. Enbridge/Handout via REUTERS

The world's largest offshore wind developer Orsted fell as much as 14% while wind turbine makers Vestas and Nordex traded down around 11% and 7.6%, respectively.

"The key message is that uncertainty is back," Alphavalue analyst Pierre-Alexandre Ramondenc told Reuters.

The U.S. renewables market is a key growth sector for several European utility companies, including Portugal's EDP Renovaveis, Orsted, and Germany's top power producer RWE, according to Deutsche Bank analysts.

"A full repeal of the IRA would depend on Congress, although significant changes might be possible," Deutsche Bank wrote in a note to clients.

While projects where construction had begun were likely to go ahead, Trump's policies could delay others such as offshore wind projects not due online until 2029 or later, the bank added.

Despite the uncertainty, Vestas and Orsted had said on Tuesday that they expected demand for green energy in the U.S. would continue to grow regardless of who won the presidency.

Angelo Meda, head of equities at Banor SIM in Milan, said he viewed Orsted's share price drop as an "overreaction", adding he took advantage of it to round up his position in the stock.

"Thinking that the trend toward green energy transition is completely over seems irrational," he said.

Nordex also remained optimistic about the U.S. onshore wind market but acknowledged potential delays in customer decisions due to uncertainty, a spokesperson told Reuters on Wednesday.

Shares in RWE, which also has wind assets, fell 6.6%, while EDP Renovaveis, traded 11% lower. The iShares Global Clean Energy ETF fell 6% to its lowest since mid-2020.

Vestas, Orsted and EDP were the biggest decliners on the pan-European STOXX 600 index.

EDP declined to comment while RWE did not immediately respond to a request for comment.