With the increasing demand in reducing carbon dioxide emissions, utilizing thermal energy storage technology, including borehole thermal energy storage (BTES), has become an efficient way to improve energy efficiency. Accurate modelling of the BTES is crucial to correctly predict the BTES performance in the building energy simulation. In this study, a large-scale BTES used for an educational building in Finland, was modelled in the IDA ICE 4.8. The BTES consists of 74 groundwater-filled boreholes with 310 m depth. The boreholes were installed with single U-tube heat exchangers. The BTES model was validated by 1.5-years measured inlet and outlet fluid temperatures of the BTES field. The results show the developed BTES model can predict the storage performance with high accuracy. During the 1.5-years validation period, the average difference between simulated and measured inlet and outlet fluid temperatures of the BTES field were within 1 °C.