This paper introduces innovative practical methodologies for evaluating the thermal performance of thermo-active pile groups. First, a streamlined approach for determining G-functions within such groups, based on the G-function of a single thermo-active pile is introduced. This is accomplished through a newly introduced thermal interaction factor for G-functions quantifying the increase in temperature when a pile is subjected to thermal interference from another pile. Subsequently, the paper proposes a method for calculating the power of piles within thermo-active pile groups when subjected to transient inlet temperatures. A thermal interaction factor for power is derived, quantifying the power reduction resulting from thermal interference due to another pile operating in the vicinity. These simplified methodologies are shown to reproduce the thermal performance of pile groups simulated using three-dimensional thermo-hydraulic analyses with excellent levels of accuracy without the associated computational cost. Finally, the proposed design process is applied to a 3 × 3 thermo-active pile group subjected to transient thermal loads, yielding accurate estimations of power, G-functions, and temperature changes of the thermo-active pile group. Overall, these simplified methodologies offer a robust framework for evaluating and optimising the thermal performance of thermo-active pile systems.