German installations of wind turbines that generate electricity are growing more slowly than necessary to prove a ``political success'' for government energy policy, according to an industry trade group.

The country needs to build enough new windmills each year to produce at least 3,500 megawatts more power, said Hermann Albers, head of the German Wind Energy Association, in Berlin today. That would demonstrate that guaranteed prices for producers are fostering a renewable-energy industry, he said. Last year, about 1,700 megawatts were added.

Germany and other European Union member states are seeking to provide 20 percent of the region's electricity by 2020 using renewable sources including wind to lower carbon dioxide emissions blamed for climate change.

Growth of new wind-turbine installations in Germany has slowed from a peak in 2002 and never reached the industry's own target. Even so, windmill manufacturers are struggling to match rising demand, especially from the U.S. and China, and because of soaring prices for steel and copper.

The German wind-energy industry forecasts a 55,000-megawatt capacity by 2020 in Europe's largest power market, more than double the existing figure. In the first half of 2008, less than a quarter of the association's annual target was installed.

The parliament voted in June to alter the country's renewable-energy law to spur the industry to control expenses and improve efficiency. The new rules included a stable, fixed price that utilities must pay for electricity generated at offshore wind turbines until 2015 and new support for last-based owners of wind farms who replace older, smaller turbines with newer, more powerful models.

Replacing the older equipment with machines five times as powerful, known as ``repowering,'' won't have an impact on installed capacity until after 2012, said Jens-Peter Molly, of the German Wind Energy Institute.

The U.S. will probably install 8,000 megawatts of wind power this year, making it the world leader, followed by China with 5,000 megawatts, Albers said.