Geothermal systems are often employed for both the heating and cooling of sustainable constructions. Energy piles (U-shaped heat exchangers inserted into the foundation piles) are widely included in these installations, whose performance is usually estimated by means of complex, time-consuming simulations already at an early design stage. Here we propose a simple methodology, where a hand calculation tool provides the condenser yield per pile meter, ground area yield and demand covered by the heat pump by specifying only building heat load and geometric characteristics of the energy piles field. Our tool is tested by assuming 20 years of operation in a hall-type commercial building in a cold climate. A validated IDA-ICE parametric study couples the heat pump evaporator operation with heat transfer processes between energy piles and soil. Various system configurations are considered and thermal storage in the soil is included. We find that the expected yield is not directly proportional to pile separation, while a smaller extraction power is favoured. Thermal storage in the soil is also confirmed to be critical. Besides our specific quantitative results, our practical guideline is qualitatively general and can be extended to any given building type and climate.