Its rows of container-sized power blocks are new pieces in the complex puzzle that is Germany's bumpy, decades-long shift away from fossil fuels and nuclear power to clean and renewable energy.

As Germans head to elections on Sunday, climate activists worry about the future of the energy transition, as the campaign has focused heavily on high power bills and the need to revive flagging industry rather than slowing global warming.

The battery plant is part of the answer to a key challenge in the green power drive: Wind and solar are clean and cheap, but output can fall short on calm and overcast days when peak demand outpaces supply.

During those "dark lulls", not uncommon in the winter, Europe's biggest economy has often had to import French nuclear or Polish coal power at high prices to avoid blackouts.

Large-capacity batteries help "fill the gap", said Tobias Badelt, spokesman of the provider Eco Stor which runs the site in Bollingstedt, a village near the port city of Kiel.

Once up and running, the facility will be able to power 170,000 homes for two hours in the morning and evening, with an installed capacity of 110 MW, among the highest in the country.

Storage systems such as this one are multiplying across Germany, with around 100 newly commissioned last year.

Such new battery plants will help "reduce electricity prices and strengthen the security of the network", said Eco Stor CEO Georg Gallmetzer.

- Surging power bills -

High energy prices have been a hot issue in the election campaign, in which politicians have argued the pros and cons of the green energy push.

While only the far-right AfD has decried the "windmills of shame", the mainstream CDU, SPD and Greens have pledged to push on with the drive towards clean, cheap and reliable energy.

The overriding goal is to help fight global warming and the threats posed by melting ice caps, rising sea levels and extreme weather events such as hurricanes, floods and forest fires.

Germany, which shuttered its last nuclear power plant two years ago in response to Japan's Fukushima disaster, has also pledged to exit coal by 2038 at the latest.

It has rapidly built up renewables -- mainly wind, solar and hydro -- which now make up nearly 60 percent of electricity produced in the country.

To ensure steady supply, especially for heavy industry, Germany long relied mainly on cheap Russian gas. But this came to an end after Russia's invasion of Ukraine three years ago, which sparked tough sanctions.

The surging power bills have hammered the German economy, which has shrunk for the past two years, a period punctuated by factory closures and many thousands of lay-offs.

Huge challenges loom as the export-reliant economy faces further headwinds, from Chinese competition to US tariffs.

Electricity demand is set to grow meanwhile when large numbers of electric vehicles hit the streets in what has been a stuttering process so far.

And billions are needed to push on with building giant power cables to take electricity from Germany's wind-swept north to its southern industrial heartland.

- Question marks -

"The energy crisis has changed our country," says the party programme of the conservative CDU, whose leader Friedrich Merz is widely expected to become Germany's next chancellor.

Merz has attacked the "ideology-driven" energy policies of centre-left Chancellor Olaf Scholz and his Green party allies.

He has also labelled the shuttering of Germany's last nuclear plants in times of crisis a "serious strategic mistake" and vowed to study a return to nuclear power.

This week he said he remains "explicitly committed" to ending reliance on fossil fuels.

But Merz also wants to delay the phase-out of combustion engine cars and rejects shuttering coal and gas plants if this endangers German industry.

"Under my leadership," he said about the various energy sources, "we will no longer abandon anything in Germany until we have decided what to replace it with."

To green campaigners, the rhetoric has raised question marks over his commitment to the national goal of carbon neutrality by 2045.

They hope the CDU will stick to its pledge that German energy policy must "combine security of supply and climate protection", but many have their doubts.

A survey by environmental group BUND found that only a third of respondents believe Merz will be able to implement successful climate policies.

BUND chairman Olaf Bandt charged that Merz's party, "with yesterday's ideas and without effective concepts... currently has no answers to the climate and biodiversity crisis".