Fibre-reinforced polymers (FRPs) offer many advantages and benefits for applications in the field of civil engineering, having been widely studied in terms of their mechanical properties and structural behaviour. However, the effects of temperature variations on these materials are often oversimplified. Structures are subject to daily, seasonal and annual thermal changes, resulting in non-uniform temperature distributions that cause thermal stresses. These fluctuations can affect the structural response more than operational loads, being one of the main factors leading to relevant deformations, when not properly dealt. This study presents a numerical study for determining the maximum and minimum temperatures that composite structures might encounter under service conditions. It focuses on the thermal analysis of hybrid FRP sandwich panels, which consist of a glass-FRP (GFRP) box structure, a polyurethane foam (PUR) core and a steel fibre reinforced self-compacting concrete (SFRSCC) top layer. Calibrated numerical models of this structure were used to investigate critical temperature scenarios, thereby elucidating how temperature gradients develop in extreme climatic conditions. The research employed a synthetic load designed in accordance with the provisions of EN 1991-1-5 and an analytical approach to calculate the critical solar radiation for specific locations throughout the year. The results of this method were compared with numerical analyses of the most extreme scenarios of the last decade, using real environmental data from different European locations. The findings demonstrate that the use the synthetic load in calibrated numerical models effectively exploits extreme temperatures on the FRP-based composite structure without leading to over-design.