Stocks in clean-energy companies held steady over the past two months — despite a backdrop of falling oil prices that, sceptics fear, could deflate the enthusiasm that investors felt for the sector earlier in the year.

The WilderHill Clean Energy index (symbol ECO on the American Stock Exchange) dipped in September but recovered last month. “It's become fairly non-correlated to the oil price — which I like,” says Robert Wilder, founder of WilderShares, the California consultancy that compiles the index on the basis of the stock prices of suppliers of non-fossil, non-nuclear energy sources.

These suppliers are now hopeful that the drop in the price of a barrel of crude oil — from more than $70 in August to $55 last month — won't deflate the boom in demand for their products. “I see no major risks unless the oil price goes below around $45,” says Michael Liebreich, chief executive of London-based New Energy Finance. “At that point some investors might lose their nerve and pull out of some projects. If that happens, stock prices will wobble.”

After a summer in which several ambitious plans for stockmarket offerings were abandoned, German biofuels suppliers CropEnergies and Verbio managed to raise some €400 million ($500 million) between them on the Frankfurt stock exchange in October.

Liebreich draws encouragement from the relative stability of the market. “There was an anomalous period, from President George Bush's State of the Union speech on oil addiction until the market correction in May,” he says, “but the market has reacted well and rationality has returned.”