## 5 Conclusions  
Under the national policy, the PVs capacity is generally set to either 3 kW or 5 kW, regardless of the power usage pattern of each household or town. If a PVs with a capacity of 5 kW generates for an average of 3.6 h per day for a month, the generated power would be 540 kWh. If a PVs with a capacity of 3 kW generates under the same condition, the generated power would be 324 kWh. In both cases, there are households that do not fit into the system. To reduce such problems and enhance the economic feasibility of the system, a case study of 905 households was done to design the proper method of deriving the optimum capacity of PVs and ESSs that is proportional to the scale of the energy self-sufficiency of 70-90%.  
Out of the 905 households in a rural farming area on the west coast of Korea, households that used an average of 250 kWh energy per month were chosen for the study, because these were the most abundant demographic. As obtaining the precise data for the number of sunless days and the exact dates of maximum/minimum energy usage is impossible, insolation data for the last 10 years achieved from Meteorological Administration Agency was used to calculate the optimum capacity of the PVs and ESSs for each household [20]. This was to prevent the decrease in economic feasibility due to the excessive capacity of the PVs and ESSs, or the decrease in energy self-sufficiency from a lack of  
Figure 6: Variation in the ESS capacity of 267.74% relative to the PV optimum with a 70–100% self-sufficiency.  
adequate capacity. A visualization of the optimized capacity for PVs and ESSs, as the desired energy self-sufficiency changed from 70% to 93%, for a household consuming 250, 350, and 450 kWh per month, was made. The optimized capacities of the PVs and ESSs for a household consuming 250 kWh per month were 2.67 and 7.15 kW, respectively, when the generation time estimated from the insolation data was considered. The energy self-sufficiency could be increased by 90% if the load usage pattern was adjusted so that it would be positioned within the generation time, which would then result in the increase of the counter-traded power. This would help to create a system to decide the capacities of the PVs and ESSs, with high efficiency and economic feasibility. With the proposed method, the optimized capacity of the PVs and ESSs could be obtained for the desired energy self-sufficiency. However, when energy self-sufficiency was near 100%, an error would occur. For those cases, we suggest the concept of a distributed network and prosumer, so that households could cover for the shortage or surplus of electricity generated.  
Generally, rural areas over the country show similar load patterns, because they tend to share a similar pattern of life. Therefore, this research would be compatible with rural areas around the nation [14]. However, the load patterns of rural areas would not have the exact match with other places, so the same approach should be taken for unknown load patterns. This research could then be expanded to the national scale. The result of applying the proposed method to other rural areas could be confirmed in our previous research on designing a carbon free renewable energy system [23]. This research would be a base project for a national project of establishing a future type of grid distributed network.  
C.Y.C.: conceptualization, methodology, writing-original draft preparation; W.L.: data curation, writing-review and editing, formal analysis; B.G.B.: visualization, validation; J.C.: resources, software; S.H.L.: data curation, investigation; S.C.W.: methodology; H.K.A.: project administration, supervision. This work was supported by the New and Renewable Energy Technology Program of the Korea Institute of Energy Technology Evaluation and Planning(KETEP), granted financial resources by the Ministry of Trade, Industry and Energy, Republic of Korea (No. 20183010014260)  
The authors declare no conflict of interest.