A study into the efficiency of wind farms has revealed that they run at a much lower capacity than previously thought.

The study, supported by Scottish conservation charity The John Muir Trust, found that wind farms are producing less than 10% capacity for more than a third of the two year study.

The study also found that low output also sometimes coincided with peak demand.

The report concluded that wind “cannot be relied upon” for a significant amount of energy generation.

The renewable industry claims that wind turbines generate at around 30% of their total capacity on average. The report found that last year, it was more like 20%

The author of the report, Stuart Young, said: “Over the two-year period studied in this report, the metered windfarms in the U.K. consistently generated far less energy than wind proponents claim is typical. Sadly, wind power is not what it’s cracked up to be and cannot contribute greatly to energy security in the UK.”

The UK currently has around 3,100 working wind turbines. A further 10,000 new wind turbines are planned in the next 10 years, and are a big part of the government’s plans to meet climate change targets.