2009 State of the Market Report
capacity factors have also generally been higher during off-peak hours, during the winter and spring, and when temperatures are mild.
Figure 47 shows average hourly wind capacity factors by load-hour percentile. Load-hour percentile helps to show how capacity factors changed in accordance with the various levels of load. The x-axis in the figure shows the tranches of data by load level. For example, ‘<25’ bars show the capacity factor during the 25 percent of 2009 hours when load was the lowest. The figure is also organized by season and region.
Figure 47: Wind Generation Capacity Factors by Load Hour Percentile 2009
West (2009) East, Central (2009) West (2006-2009) East, Central (2006-2009)
25-49 50-74 75-94 95-99 Summer
25-49 50-74 75-94 95-99 Winter
Load Hour Percentile
Wind output (reflected in the capacity factors) is generally negatively correlated with load, particularly in the summer. Capacity factors are lowest when the output is most valuable. The spread between western and eastern capacity factors is larger in the winter than in the summer, but the difference narrows at the highest load levels. These results are consistent with the Midwest ISO’s analysis supporting the reduced capacity credits for wind resources from 20 percent to 8 percent.