Electricity generators require significant maintenance to ensure they are available to run when called on. The owners and operators of electricity generators typically perform maintenance during the low-demand seasons (spring and fall). For example, the chart shows recent historical maintenance data from Independent System Operator – New England (ISO-NE). At certain times of the year, 20-25% of its generating capacity may be out on scheduled maintenance. Maintenance reduces the likelihood that the generators will be unable to run in high-demand periods (summer and winter) when needed.