Efficient and robust thermal management is a key prerequisite for reproducible vehicle testing and energy-efficient powertrain operation. Conventional thermal operating strategies are commonly based on temperature setpoints and often neglect the dynamic relationship between heat input, system inertia and transient operating conditions, leading to increased energy demand and limited reproducibility under highly dynamic driving cycles. This paper presents a simulation-based development and evaluation of a heat-quantity-based thermal operating strategy for automotive applications. Instead of controlling the system solely via temperature targets, the proposed approach utilizes accumulated and instantaneous heat input as central decision variables for thermal control, explicitly accounting for thermal system dynamics. The investigation is conducted using a detailed vehicle simulation model including a physically-based thermal system representation. An innovative decentralized thermal architecture enables controlled thermal coupling between the coolant circuits of the battery, power electronics/electric machine and internal combustion engine via multiple three-way valves. This allows a situation-based, bidirectional waste-heat transfer between subsystems and provides the basis for the proposed operating strategy. The heat-quantity-based strategy is systematically compared to a conventional temperature-based reference under transient driving scenarios. Evaluation criteria include temperature control accuracy, thermal stability, energy input to the cooling system and reproducibility of thermal boundary conditions. Simulation results demonstrate improved temperature trajectory control and reduced unnecessary energy input, highlighting the potential of heat-quantity-based thermal management strategies for automotive applications.The strategy is designed for later integration into a prototype vehicle, experimental vehicle validation is subject to future work.