This paper deals with the methodology of sizing a sustainable heating-cooling system, which consists of a heat pump, thermo-solar collectors and underground water tank, as a seasonal thermal energy storage. The methodology is based on successive and coupled energy balancing of all system elements for time intervals of 1 h, during one representative calendar year. The starting assumption for the calculation contains the size of the underground storage tank, its burial depth, the size of building that is heated or cooled, and the temperature of air in the building. To make the calculation, it is necessary to know the so-called dynamic boundary conditions during a calendar year, comprising the following: hourly outdoor air temperature, hourly insolation (received solar radiation energy) and hourly temperature of the ground surface. The calculation result is the size of the thermal-solar collectors surface, which will ensure the sustainability of the system, i.e. it will ensure that after one year of operation this system is restored to the original conditions, without disturbing natural environment in which it is used. For the purpose of presenting the methodology and checking the influence of individual parameters of the system's operation, a numerical simulation was performed and required surfaces of thermo-solar collectors were determined for one of the system models. Analyses of the influence of the underground tank burial depth, tank size and the initial water temperature in the tank on the required size of thermal-solar collectors were also performed. (C) 2014 Elsevier B.V. All rights reserved.