The origins of parasitic heating for photovoltaic (PV) technologies based on silicon, perovskites, and their combination in monolithic tandems are investigated. To quantify heating losses, the cooling score (CS) as a new simple metric, representing the percentage of incident solar irradiance not contributing to module heating is introduced. This is a function of both the optical structure and power conversion efficiency (PCE) of the PV modules and allows a fair comparison between different technologies under identical performance-evaluation scenarios. Silicon single-junction devices have the lowest CS due to their low bandgap (causing significant carrier thermalization losses) and their use of light-trapping structures to increase their PCE which also undesirably increases parasitic absorption of sub-bandgap photons. Conversely, perovskite single-junction devices show the highest CS in all studied performance-evaluation scenarios thanks to their wider bandgap and high absorption coefficient, enabling absorption of all solar photons that may contribute to the photocurrent without requiring light-trapping structures. While perovskite/silicon tandems minimize thermalization losses, they also usually employ light-trapping structures in their bottom cell to increase their PCE, which lowers their CS. Through simulation and outdoor experiments, it is demonstrated that an efficient module-cooling environment may significantly suppress the detrimental effects associated with a low CS.