The degradation of solar photovoltaic (PV) modules over time, aggravated by defects, significantly affects the performance of utility-scale PV parks. This study presents a quantitative assessment of the power loss from module defects, and evaluates the cost-effectiveness of replacing defective modules at various stages of degradation. A module test site was established in Norway with six different defects, and continuous thermographic monitoring, combined with light IV measurements and electroluminescence (EL) imaging, provides partial support for further calculations on the long-term effects of the defects. The cumulative module energy loss is calculated over a 25-year park lifespan under both Norwegian and Chilean environmental conditions, the latter representing higher solar irradiation levels. The energy gain from replacing the defective modules at various stages of degradation is compared to the costs of replacement, both for infant-life failures and mid-life failures. Minor infant-life defects of 1\% power loss are likely not beneficial to replace in low-irradiation regions like Norway. For Chilean conditions it can be cost-effective, but primarily if the module is replaced around mid park life, which gives a larger yield when replaced with a new module. For more severe defects of 10\% loss the replacement gain is above the replacement cost for high-irradiation locations, and replacing the 33\% power loss defect is cost-effective for both locations, even when discovered late in the park lifetime. Mid-life defects are primarily beneficial to replace in high-irradiation locations.