## 5 Conclusions  
Most of the systems with one type of renewable energy source consist of PV power generation and an ESS. Although the ESS is used as an auxiliary device in a PV power system, since it lacks the ability to generate constant power every day, there are obvious limitations to the stability of the power supply because there may be a situation (e.g., the rainy season) where solar energy cannot be received for a long period of time. In order to increase the stability of the power supply, it is essential to introduce a second energy source that can always secure a constant amount of power generation. A second energy source should be used as a base power source responsible for the minimum load in order to minimize wasted power generation.  
In this paper, a design technique for constructing a renewable energy-based power system based on the customer's power load was proposed. This design method introduces a second renewable energy power generation source in charge of the base load by improving the referenced design method with one type of renewable energy power source. The fuel cell was introduced as the base power source, and the main power source was set as PV power generation and an ESS for the middle and peak loads.  
The average daily load of the area targeted in this study is 2197 kWh, and the minimum load is 32.28 kW in a year.  
The capacity of the fuel cell was set to 35 kW, which is a rated capacity higher than the base load, and the amount of daily power supplied by the fuel cell is 840 kWh. The capacity of the PV and ESS is designed to supply power as much as 1357 kWh, which is middle and peak loads that the fuel cells cannot supply. The designed capacities of the fuel cell (base power sources), PV power, and ESS were set to 35 kW, 442.15 kW, and 1375.96 kWh, respectively. The total cost of the designed power generation facility was estimated to be USD 1,243,874.70. It was determined that with using a base power source, the cost would be lower than the system by a margin of USD 533,849.15. Moreover, it was calculated that there would be a cost reduction of approximately 30.03%.  
In conclusion, when \(x\) (the system unit cost of the base power source (fuel cell)) is equal to or less than \(y\) (the PV power system unit price) + 2.96 z (the unit price of the ESS), the stability and economic feasibility of a power system with a base power source are secured. It should be noted that the 2.96 of the proposed criteria in this study is a value driven from a general PV system and its components'  
Figure 7: Visualization of the criterion under which a system with base power source is economical.  
(inverter, ESS, etc.) condition. Therefore, this figure could be changed according to technological advances or environmental changes. Equations (10) and (11) explain the elements that make up the constant 2.96 and their correlation.  
It would be possible to promote the supply of economically feasible renewable energy through the proposed criterion of adopting a base power source and the system design method proposed in this study. The unit cost of the ESS and fuel cells, which are still high, will gradually decrease with the development of technology, which would also support the promotion of the spread of renewable energy. The lowest load during the year should be determined as the base load with different seasonal, monthly, and daily changes. It would then be necessary to calculate the capacity of the base power source according to such changes for future works.  
S.H.L.: Conceptualization, methodology, writing--original draft preparation; W.L.: data curation, writing--review and editing, formal analysis; J.H.H.: investigation, data curation; B.G.B.: visualization, validation; J.C.: resources, software; H.K.A.: supervision. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.