Clean-energy stocks have slipped back over the past two months after hitting record highs in the summer, when oil price increases to $70 a barrel alerted investors to the potential value of stocks in alternative energy sources.

Credit: WILDERSHARES

The Wilderhill Clean Energy Index — whose symbol on the American Stock Exchange is ECO — tracks the performance of companies whose businesses rely significantly on energy sources other than fossil fuels or nuclear power.

And as oil prices have fallen this autumn, the index has taken a hit. Nonetheless, says Robert Wilder, the former political scientist whose company runs the index, money continues to pour into funds set up to track it. “That's an indication of strong, ongoing interest in the sector,” he says.

Big movers in the past two months included California microturbine-maker Capstone Turbine, whose stock halved in value as the speculative interest that had boosted its price fourfold in the summer wore off.

However, stocks in solar-panel supplier Evergreen Solar of Massachusetts advanced by 50%, to almost $12, on powerful global demand for its photovoltaic cells.

If oil prices fall back further next year, Wilder suggests, speculative investors may start to frown on alternative energy sources. But even with oil at, say, $50 a barrel, he says the sector could enjoy sustained growth in demand for its products in the medium term.