The potential for high energy efficiency of ground source heat pump systems has resulted in rapidly growing numbers and sizes of such installations. The energy efficiency potential of a ground source heat pump system can be further enhanced by optimized design of the ground collector and storage system. A key parameter is the brine temperature and this paper evaluates some methods for its calculation. We also introduce three key numbers to characterize ground storage systems and their load situation. Finally, the effects of different building load patterns and borehole system designs and configurations on brine temperature are analyzed.