The growth of electromobility emphasizes the need for good charging solutions. Electric vehicles can be charged either while standing still (static charging) or while in movement (dynamic charging) by using electric road systems (ERSs). In both cases, significant power levels are used to transfer energy to the vehicles, thus requiring a thermal design that is able to handle the losses generated while charging. This paper proposes a thermal model to assess the temperatures inside a particular type of ERS: an Alternating Short-Segmented ERS (ASSE). The thermal model is calibrated and validated against measurements on a test track with a full scale prototype. Measurements show that the ASSE can supply charging power in excess of 90 kW continuously. Peak power of 180 kW can be sustained for 10 min before the ASSE is at risk of overheating. Nonetheless, after high-power charging, the temperature of exposed surfaces may entail a safety hazard and the model can be used to design and evaluate potential corrective measurements to mitigate this.