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Module performance data collected at two sites are analyzed in order to identify the respective magnitudes of seasonal annealing and degradation in comparison to spectral effects. It is demonstrated in this paper that at one site (Loughborough, UK) the spectrum dominates and very little seasonal annealing is observed. In contrast, at the other site (Golden, US), half of the seasonal variation can be attributed to spectral changes while the other half must be attributed to thermal annealing of defects. Differences between multi-junction categories are investigated and it is shown that single-junction devices exhibit a greater seasonal annealing than multi-junctions, while the latter tend to be more influenced by spectral effects.