Rising oil prices and increasing strict emission legislation force vehicle manufacturers to reduce fuel consumption of future vehicles. In order to meet this target, the process of converting fuel into useable energy and the use of this energy by the different energy-consuming vehicle's subsystems have to be examined. Vehicles' subsystems consist of energy-supplying, energy-consuming, and in some cases energy-storing components. Due to the high complexity of these systems and their interaction, optimization of their energy efficiency is a challenging task. By introducing individual operational strategies for each subsystem, it is possible to increase the energy efficiency for a specific function. To further improve the vehicle's overall energy efficiency, holistic control strategies are introduced that distribute the energy between the subsystems intelligently. To exhaust the whole potential of a holistic control a certain number of degrees of freedom between energy supply and consumption are necessary. This is particularly relevant for subsystems that consume large amounts of energy. To solve this problem, a new methodical approach is being presented. This approach is applicable to every energy-consuming subsystem and describes requirements and advantages of unified system architectures as well as a procedure to construct energy-efficient operational strategies, with the air conditioning system being used as an example. To support the methodical approach, a simulation model has been created. The model describes the energy flows among energy suppliers, energy reservoirs, and energy consumers of given subsystems and in particular of the air conditioning system, depending on the vehicle's environment and a selected driving cycle.