Venture backers of European clean energy startups reaped a 55 percent annualized return on their investments from 1998 to 2007, the London-based research group New Energy Finance said Tuesday.

The analysis, which was commissioned by the European Energy Venture Fair to take place this weekend in Zurich, looked at returns earned by 37 venture capital and private equity investors in 129 early stage companies dealing in low-carbon technologies such as renewable energy, fuel cells, power storage since 1998.

Of the 129 Europe-based portfolio companies that participated in the overall analysis, 15 IPO’d and 10 were sold to trade buyers, the research group said. Twenty-one companies raised more money with subsequent funding rounds.

At the same time, nearly an equal number of companies, 19, had been “written down” or raised smaller, subsequent funding rounds.

Over the same 10-year period, the 37 investors plowed €283.6 million ($396.1 million) into the portfolio companies and earned exit returns of 1.4-times funds invested. For non-exit companies, returns were 1.2-times total funds invested, according to New Energy Finance.

The 129 companies included in the analysis comprise about half of the clean energy companies in Europe and Israel, including non-European Union members, according to New Energy Finance.

The group included private equity investment in public companies by VC firms in its analysis. But it excluded buyouts, project financing by private equity investors, and general over-the-counter investment, said New Energy Finance CEO Michael Liebreich.