By Kate Abnett

BRUSSELS (Reuters) - The growth of solar power installations in Europe slumped to just 4% this year after years of double-digit increases, industry data showed on Tuesday, heightening concerns about the continent's clean energy transition.

The findings bode ill for the European Union's plans to rapidly expand its use of renewable energy - a central pillar of the bloc's efforts to fight climate change and end its reliance on Russian fossil fuels.

The amount of solar energy installed in the EU had jumped by more than 40% year-on-year in both 2021 and 2022, and by more than 50% in 2023, industry association SolarPower Europe said.

This year, 65.5 gigawatts (GW) of solar was installed, a record high amount, but just 4% more than what was installed in 2023.

"The growth is substantially slowing down," SolarPower Europe CEO Walburga Hemetsberger told Reuters.

The EU needs to install 70GW of new capacity each year to meet its 2030 green goals.

The group scaled back its growth forecasts as a result of this year's slowdown. Its central outlook still expects Europe to have 816GW of installed solar capacity by 2030 - enough to meet Europe's green goals, and a jump from 338GW today.

But for the first time, the group's "low" outlook would now see Europe miss its targets to expand renewable energy.

The slowdown reflects the fact that many of Europe's power grids have not been upgraded to handle a bigger share of renewable energy. Solar projects often have to wait years to be connected to the grid, or their energy production is curtailed - meaning it is wasted - to avoid overwhelming a struggling grid.

"What we see more and more in different countries, all over Europe, is that we're running into grid bottlenecks," Hemetsberger said.

The data, shared exclusively with Reuters, showed home rooftop solar installations decreased year-on-year by nearly 5GW, to 12.8GW in 2024.

SolarPower Europe attributed that to energy prices falling from their peak during Europe's 2022 energy crisis - weakening the incentive for consumers to buy solar panels to cut their bills.

Investments in European solar projects also fell, for the first time this decade.

(Reporting by Kate Abnett; Editing by Lincoln Feast.)