It is known for a-Si:H modules that seasonal variations in performance occur because of the effects of meta-stable degradation mechanisms. These changes in Standard Test Condition (STC) performance are driven by the history of exposure to light and temperature with different time constants and thermal annealing activation energies. The present study provides a link between performance changes following varied indoor controlled conditioning and during outdoor exposure through different seasons. Varying exposure conditions were simulated indoor at different light levels and temperatures as functions of time. During these controlled conditioning experiments the extrapolated STC performance was found to change significantly, and coefficients determining the change in efficiency as a function of light and temperature exposure have been estimated. Using these coefficients the module performance has been modelled as a function of real irradiance and temperature variations over a year leading to a good match to the actual measured performance over the same year. (C) 2007 Elsevier B.V. All rights reserved.