**4. Conclusions**

In this study, several experiments were conducted with different operation modes to sugges<sup>t</sup> optimal operation modes of residential BESS with balcony PV in Korea. The experiment apparatuses were a 2.016 kWh BESS, a 1.2 kW balcony PV, and an electric fan. The operation mode of BESS was divided into four types according to PV power supply priority and battery charge source.

The results show that if PV power was supplied to the load first, the charge/discharge of the battery was determined by the load (mode 1 and 2). However, when PV power was supplied to the battery first (mode 3 and 4), PV power was supplied to the load after the battery was fully charged. Furthermore, if the battery was only charged with PV, there was a risk of full discharge in the absence of PV (mode 1 and 3), but charging with the grid prevented this (mode 2 and 4).

Based on the characteristics of each operation mode, it was determined that mode 4, in which PV power preferentially charges the battery and then charges the battery with PV and grid power, is appropriate for the BESS operation mode of individual households of apartment houses. However, some functions need to be added to ensure applicability of residential BESS to individual households of apartment houses in Korea. First, the load connected with BESS should be the total power consumption in the household, not the power consumption of home appliances connected to a separate load line. Second, the battery must maintain the minimum voltage using the grid power before reaching an unusable battery condition. Third, it should be possible to set the voltage range or time for PV charge. By satisfying the above conditions, BESS with balcony PV is expected to be used efficiently for individual households of apartment houses in Korea. Furthermore, it is believed that it will contribute to zero energy in houses by improving the energy independence rate of households.

**Author Contributions:** Methodology, data collection and analysis, visualization, validation, writing— original draft preparation, J.E.; conceptualization, methodology, writing—review and editing, supervision, Y.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20172410104720).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** To the best of our knowledge, the named authors have no conflict of interest to declare, financial or otherwise.
