Over the past two decades, a substantial amount of research has investigated the effects of thermal loads associated with the geothermal operation of energy piles on their mechanical response. Based on this research, consensus about the need for considering the effects of thermal loads on the geotechnical and structural design of energy piles has been achieved. However, the understanding of the influence of thermal loads on the mechanical response of energy piles has prevented the determination of when the effects of these loads should be considered in performance-based design, e.g., only when addressing the deformation (at serviceability limit states) or also the failure (at ultimate limit states) of such foundations. Looking at this challenge, this paper presents an investigation of the role of thermal loads in the mechanical response of energy piles to provide a theoretically based approach for the geotechnical and structural performance-based design of such foundations. The main conclusion that can be drawn from this study and the discussed ductility-based design approach is that thermal loads cause negligible effects at ultimate limit states from both a geotechnical and a structural perspective, while they cause significant effects that should be considered at serviceability limit states.