the difference between the peak of the load duration curve and the residual load duration curve represents the generation capacity that on average is not required due to the presence of variable wind energy the difference between the black and the blue curves shows the minimal generation capacity over all samples which is not needed in the last edition of the world energy outlook iea c the iea assumes that the annual peak demand is normally distributed around the mean the capacity credit is then calculated based on the difference between the peak demand and the point one standard deviation above the residual peak demand