Some solar output forecasts are too optimistic.

Projects greater than one megawatt in size generated 5% to 13% less output during the period 2011 through 2020 than predicted by P50 output forecasts, according to kWh Analytics in its latest “Solar Generation Index” report in late October.

The performance was worse in southern states than in northern states. Projects in the Pacific Northwest performed better than expected. Those in the southwest showed the biggest gaps.

The latest numbers are based on 350 operating solar projects with a total capacity or more than 10,000 megawatts and performance over the period 2011 through 2020. The calculations are weather adjusted.

Output has become noticeably worse compared to forecasts since 2016.