Germany installed 2073MW of new onshore wind capacity in the first three quarters of 2018, a 50% fall on the same period last year, according to federal government figures.
A total of 646 new turbines were erected across the country by the end of September, the data shows.
The region of Lower Saxony had the most new capacity with 606MW, a 40% drop on 2017.
North Rhine-Westphalia was in second place with 325MW and Brandenburg third with 232MW of new onshore wind.
The overall federal expectation is that less than 3GW will be installed by the end of 2018.
German wind energy association president Hermann Albers said: “In the first three quarters of this year, wind turbines with 2073MW have been connected to the grid.
“Although experience shows that additional momentum is expected in the last quarter, the additional construction in 2018 is likely to remain below industry expectations. The market slump is thus more dramatic than expected.”
The association said that plans for new onshore tenders in 2019, 2020 and 2021 are “therefore enormously important”, but would be “too late to effectively smooth out a collapse in new construction in 2019 and 2020”.