On Friday, the turbines in Bilbster, near Wick, could only muster up enough energy to boil little more than 180 kettles.
The Scottish Government has a goal of generating the equivalent of 50% of the country's electricity from renewable sources by 2015 and 100% by 2020.
But critics called for a change of policy as new figures have revealed output from wind farms changed dramatically day-to-day over the summer.
On June 23, wind farms contributed 95,206MWh (megawatt hours) - but two days later the total fell to just 6293MWh. Over one week-long period at the height of the heatwave, output crashed. Total generation was 22,742MWh on July 21, only to fall to 9615MWh over the next six days.
Linda Holt, of Scotland Against Spin, said: "Wind is completely variable and completely unpredictable which means it can only ever be an adjunct to a proper energy supply that is steady and predictable."
A Scottish Government spokeswoman said renewable targets were "ambitious, but achievable".