Understanding the degradation of a photovoltaic (PV) system is crucial for predicting the behaviour of a PV system or acting quickly in case of strong PV module degradation. A novel concept with Wi-Fi devices for monitoring the substring voltages of PV modules is presented and case studies of PV modules' substring voltages during operation of the PV system are reported on. A new electronic device called Jubomer was custom developed to fit into the junction box of a PV module and to measure the voltages of each module substring. The analysis of substring voltages shows that this methodology can reveal problematic modules without conducting a comprehensive testing procedure on disconnected PV modules. The results of the voltage measurements are also linked with the detailed analysis of a PV module using an electroluminescence imaging technique, which verifies the proposed concept.