Abstract The outdoor operation and monitoring of amorphous silicon (a-Si) solar modules present unique features when compared to the more traditional and quite well understood operation of the crystalline silicon (c-Si) technology. The peculiarities of a-Si contrast to such extent with those of c-Si solar cells that in the field, while the former performs better during summer, the latter is more efficient in winter. Concepts usually applied to describe phenomena in c-Si devices are often inadequate to describe the performance of a-Si cells. When looking at module performance, the fill factor (FF) can be regarded as one of the characteristic photovoltaic quantities of major interest. Under outdoor illumination, cells are seasonally exposed to different solar spectral contents and intensities, which vary considerably from summer to winter. The FF depends on both the quality (spectrum) and quantity (irradiation) of the incident light. In this context, we report results showing spectral effects on the FF of amorphous silicon solar modules deployed outdoors. While “blue” spectra improved the FF of a-Si devices, the contrary was observed for “red” spectra. The voltage-dependent spectral response of a-Si devices is also described and quantified. Our results reveal that a-Si modules can perform quite well at low irradiations and mainly diffuse spectra. We, thus, conclude that in system sizing programmes, the performance of a-Si modules should be treated more precisely with respect to spectra, to reveal their true operational characteristics and advantages.