Borehole thermal energy storage (BTES) has reduced energy usage by supplying large portions of the heating and cooling demands in buildings and industry. System modeling is used for design and preliminary assessment of expected savings from a BTES investment. The present study shows that inaccuracies in BTES model results or replacing the BTES model with a computationally less demanding model with deviating results has small impact on system level parameters such as the total Seasonal Performance Factor (SPFtotal). Here, a BTES model requiring relatively low computational effort is presented and compared to a well-established model based on g-functions. The presented model applies connected thermal masses and resistances and assumes an average behavior of all boreholes. An example case of a hospital building is simulated with the building performance simulation tool VKB-sim, using the two different BTES models. In addition, changes in ground parameters of up to 30% and borefield configuration are considered. In the energy system model, SPFtotal differs by only a few percent even when there are large deviations in results from the BTES model alone. Hence, the computational effort may be reduced if the aim is estimation of overall system performance or savings.