As an alternative energy technology, Ground Source Heat Pump systems (GSHPs) offer greener advantages over conventional heating and cooling systems. This has led to their penetration not only in residential buildings but also increasingly in commercial and industrial ones as well. However, longer payback periods, lesser return on investment, and higher upfront costs often make GSHPs unappealing compared to their conventional alternatives. Hybrid GSHPs offer a solution to decrease the initial costs and to make GSHPs more economically viable. Hybrid systems employ ground heat pumps for providing base load needs, and conventional systems for supplementing peak demands. The capacity of a GSHP in a hybrid system is usually determined by following rough rules of thumb, and then calculations are made to test for economic viability. The current process for determining a GSHP capacity is neither mathematically rigorous nor optimized. In this study, a rigorous mathematical, computational approach to size the GSHP within a hybrid system is presented. The methodology is tested for ten cases from residential to commercial and industrial buildings. Using this methodology can result in significant reductions in initial costs of installation, payback period, and operation costs, when compared to following rules of thumb or using non-hybrid systems. In most cases, when optimization is performed, the GSHPs meet a very large portion of the heating and cooling demand of a building.