Ground source energy systems provide low-carbon heating and cooling to buildings, but their efficient deployment requires a reliable estimate of their thermal performance. A simplified methodology is presented to determine the thermal performance of thermo-active piles when heating or cooling loads are specified with either inlet pipe temperatures or imposed heat fluxes. The proposed methodology avoids computationally expensive 3D analyses and the explicit simulation of heat exchanger pipes, relying instead on 2D thermal analyses. When the heating or cooling of a thermo-active pile is assessed by imposing inlet pipe temperatures, the proposed methodology allows the determination of the power of pile per unit length. Conversely, when heating or cooling loads are specified via extracted or injected heat fluxes, the inlet and outlet fluid temperatures, as well as average temperatures at pile wall, are determined. The proposed methodology has been shown to reproduce accurately the thermal performance of thermo-active piles modelled using 3D analyses where heat exchanger pipes are explicitly simulated, considering different patterns of heating and cooling cycles. The application of the proposed methodology to the case of a real thermo-active pile is demonstrated by comparing its predicted thermal performance with the results of a well-documented field thermal response test.