Depending on its local characteristics and other factors such as climate conditions and soil thermal properties, the ground may absorb a large part of the solar radiation received by the earth’s surface. To a certain depth, the ground can act as an indirect heat energy source since it is able to collect, store and transmit energy. Buildings’ thermal performance can benefit from using this indirect solar energy through earth-coupling strategies. EnergyPlus’ Basement Auxiliary program is a useful tool for thermal simulation of buildings with direct ground contact. However, input data such as terrain tilt or orientation is not available in this software. This limitation means that the ground heat transfer calculations offered by this program always assume that ground-integrated buildings are placed on horizontal terrains. Through the use of a mathematical model, the authors explained how undisturbed ground temperatures under a flat terrain are different from those under sloping terrain. Consequently, the authors argued that terrain inclination and orientation should be included as parameters for simulations of the ground heat transfer of buildings. Two elements that Labs’ equation uses to calculate ground temperature are solar radiation aspects as the mean annual temperature of the surface of the soil (Tm) and the annual range of the temperature wave at the soil’s surface (As). In this paper, the authors analysed the impact of using the appropriate solar radiation values for calculating ground thermal potential of slopes, which was done by using the corresponding solar radiation data for Lisbon surface tilts from 0° to 60°, with 5° intervals and with zero Azimuth. The resulting analysis showed that, as a consequence, Tm and As values were affected, proving that ground temperatures below slopes were different from those registered under flat terrains. The authors demonstrated that all terrain inclinations provide higher annual ground thermal potential than equivalent flat terrain, and that between flat and sloped terrain an intermediate ground temperature zone was produced. The authors therefore concluded that for this location, 30° to 40° slopes were the best terrain inclination to achieve maximum ground thermal potential.