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The energy potential of geothermal reservoirs mainly depends on thermal regime, geological structure, and petrophysical properties. However, practical issues have a main importance in the planning, such as the kind of energy utilization, the design of the geothermal doublet, or the vicinity of energy end users. The evaluation approach here described tackles and integrates the different aspects. The approach is tested in the medium-enthalpy geothermal reservoir of Guardia Lombardi (Campania, Italy). The surface area of the entire study region is about 42 km by 28 km. It is characterized by specific surface heat flow of up to 90 mW m-2 and geothermal gradients of about 58 K km-1. The geothermal target is the shallow water Apulian Platform, a fractured and possibly locally karstified carbonate layer, dissected by a number of steep faults. The complex geological structure has been interpreted from several seismic profiles and hydrocarbon exploration wells. A numerical model was set up describing the relevant regional advective and conductive heat transport processes in the reservoir. A set of borehole-temperature data was used for its calibration. As simulator we used the code SHEMAT-Suite. First, we investigate the natural flow and temperature conditions in the entire reservoir. Second, we choose several sub-models, based on temperature and flow rates as well as on social decision parameters such as vicinity to villages or natural reserve restrictions. For these sub-models we assess the rates of energy production and sustainability for various utilization scenarios. Relevant aspects, beside infrastructure and natural reserve restrictions, are: (a) borehole locations and distance between injector and producer; (b) drilling depth vs. estimated temperature; (c) influence of preexisting fracture and fault systems; (d) stimulation.