Power devices are among the reliability-critical components in the Photovoltaic (PV) inverter, whose failures are normally related to the thermal stress. Therefore, thermal modeling is required for estimating the thermal stress of the power devices under long-term operating conditions of the PV inverter, i.e., mission profile. Unfortunately, most of the thermal models developed for the power device are not suitable for a long-term thermal stress analysis (e.g., days to months), and there is usually a trade-off between the model accuracy and the computational efficiency. To address this challenge, a reduced-order thermal model for PV inverters is proposed in this paper, where the model simplification is based on the thermal impedance characteristic and the mission profile dynamics. The modeling accuracy is evaluated by comparing the estimated thermal stress with the experimental results from a PV inverter test-bench, where daily mission profiles with various dynamics are tested. According to the results, the proposed method offers a relatively high model accuracy (similar to the full-order thermal model) while the computational efficiency is improved significantly, making it suitable for long-term thermal stress modeling applications.