Thin-film silicon (Si) related photovoltaic (PV) modules suffer performance deterioration owing to outdoor exposure. We investigate the performance degradation and seasonal variation of hydrogenated amorphous Si (a-Si: H) related PV modules. The normalized maximum power (P-MAX) of the single-and multi-junction modules tested herein decreased greatly and then stabilized after similar to 3 or 4 years of outdoor exposure. In summer (August 2016 and 2017), the P-MAX of all a-Si: H related PV modules increased by 2.0%-3.8% over the P-MAX in winter (December 2015 and 2016). In addition, the a-Si: H related PV modules show higher performance ratio (PR) in summer and lower in winter. These results are due to thermal annealing and light induced degradation effects in a-Si: H related PV modules. The a-Si: H related PV modules' PR two months after outdoor installation is lower than that before installation, which is explained by initial light exposure induced degradation.