An unexpectedly cloudy day can cause solar generation to dip below expectations, and in consequence, other generators will need to compensate with additional output. One possibility is that fast acting gas combustion turbines would fill this generation gap, and while this is suitable solution from a reliability standpoint, a more fuel efficient combined cycle generator might have been able to compensate for the reduced solar output given enough prior warning. This is an example of the cost of a solar forecast error, that is, a less than optimal set of generators meets demand.