Estonia is among the countries with the most energy-efficient Near Zero Energy Buildings (NZEBs) in Europe, i.e. their energy consumption is the lowest. From the energy management prospects, on one hand, the rising price of electricity and decreasing the feed-in prices, and on the other hand, the decreasing price of battery energy storage system (BESS) have made the self-consumption of NZEBs profitable. This paper develops a simple tool to study the optimal sizing of BESS and increase the profitability of NZEBs. The BESS sizing is done considering the installed PV capacity in the NZEB, the energy consumption profile, and the local electricity prices. The results of this paper demonstrate to the NZEB owner and planers that in case of self-consumption, investment in PV systems could be more profitable when it is combined with BESS. Simulation results are presented to validate the proposed BESS sizing tool and analyze the sensitivity of results to changes in electricity and battery system price.