Fluctuations in demand and the possibility of unscheduled outages make it essential to have plant in reserve. This is always the case regardless of what kind of generators contribute to the overall capacity. Where a system includes intermittent sources, however, the required level of reserve capacity will be greater. This additional capacity is not specifically dedicated to covering for loss of intermittent supplies so that an investor in wind power, say, would not need to concern himself with this. Nevertheless, it does represent an additional cost on the system and is, therefore, a hidden subsidy for intermittent suppliers.

For wind turbines, back up is most obviously needed for those times when peak demand coincides with very low wind conditions. When the percentage of total capacity provided by wind (so-called wind penetration) is small, any shortfall can be readily made up by boosting the output of thermal generators or increasing imports. As penetration increases however, it becomes progressively more difficult to bridge the gap. Recent modelling indicates that at 20% wind penetration, the UK would need backup equivalent to almost 50% of the firm wind capacity defined as the installed (i. e. nameplate) capacity multiplied by the average availability.17