Renewable and green energy companies have been drowned in cheap oil, and the exchange-traded funds that own them have been punished along with the rest of the energy sector.

In the US, Guggenheim Solar and PowerShares WilderHill Clean Energy are both down more than 30% in the past year, about the same as the Vanguard Energy ETF's 32.6% tumble. But that is a mistake - one that can prove to be very profitable for investors. Oil dominates the energy sector - the Vanguard Energy ETF has a 22% stake in ExxonMobil, for instance. But oil is rarely used to generate electricity, which is the aim of most green energy companies. What is more, green energy stocks are technology, not commodity, stocks: they sell the technology needed to harness and generate energy.