Several works report on seasonal fluctuations of power production of amorphous silicon (a-Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler–Wronski effects. It is hence difficult to assess—for a given location and climatic conditions—which one has the largest impact. By means of a straightforward approach based on two sets of single-junction a-Si photovoltaic modules (stored indoors/exposed outdoors) and on two different I–V measurement set-ups (indoor and outdoor), we were able to separate the different contributions to this phenomenon. For the test-site of Lugano, seasonal oscillations account for performance variations of a-Si of ~10% (±5% around an annual average value with a minimum around the mid of January and a maximum around mid-July). The time-phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a-Si with an amplitude corresponding to 10.5% (± ~5.2%). Moreover, the influence of spectral variations on the outdoor performance of a-Si (and for comparison of c-Si) was modeled, and the experimental data were found to be in excellent agreement with the theoretical simulation; (ii) the Staebler–Wronski effect has a slightly lower influence with an amplitude of ~8% (±4% with a minimum at the middle of February and a maximum around mid-August). Because of the position (46°N) and average climatic conditions (southern Alpine climate) of Lugano, these observations are possibly representative of a large part of continental Europe. Copyright © 2012 John Wiley & Sons, Ltd.