In their ‘Cuxhaven Appeal 2.0’, the group further demands at least 30GW installed by 2035 — an increase on the government’s 2014 target of 15GW by 2030.
They also asked for more research and development funding, an improved grid system, better-maintained and expanded ports, and for a drive to boost competition in the sector.
These changes would help Germany boost economic development and help it meet its climate targets.
The group behind the Cuxhaven Appeal comprises ministers from Lower Saxony, Schleswig-Holstein, Mecklenburg-Vorpommern, Hamburg, and Bremen, the mayors of 12 cities and towns in northern Germany, the president of industry body Offshore-Windenergie, and IG Metallkuste's district manager Mainhard Geiken.
They had initially called for government action on offshore wind in 2013.
But the "considerable increase in the production capacity" of renewable energy sources — as evidenced by successful zero-subsidy bids for projects in Germany’s first competitive tender in April — necessitated "intensive efforts to expand the network", the coalition wrote.
As of 1 September 2017, Germany had 4.56GW offshore capacity installed with a further 16.61GW planned by 2030, according to Windpower Intelligence, the research and data division of Windpower Monthly.
If these projects in the pipeline are completed, Germany would have a total offshore capacity of 22.31GW, not including repowering or decommissioning.
This increased capacity would help boost economic development and help the country meet its targets of reducing its 1990 greenhouse gas emission levels by between 80% and 95% by mid-century — including a reduction of 55%-56% by 2030, the group wrote.
The industry currently supports around 20,000 jobs, according to the letter.
But German manufacturers Nordex and Senvion have announced job cuts from across their European business — with a likely concentration in Germany — in cost-cutting exercises.
Offshore expansion would also maintain jobs, trade union IG Metallküste said.
Meinhard Geiken said: "We need a stronger expansion of wind power in order to achieve climate protection targets, but also to secure employment and locations in Germany that all the signatories agree on."