## 4 Development of the Profit Rate of an ESS-Connected PV Analysis Method
### Development of Economical Evaluation Method
The PV, ESS, PCS capacities of each case were decided by the following rules: the capacity of the PV system needs to be equal or greater than capacity of PCS, because PCS does not operate with PV and ESS simultaneously; the capacity of the ESS is set so that the depth of discharge would be below 90%. The capacity of the PV can be freely decided, but the capacity of PCS must follow the standard. Therefore, in cases 2 and 4,the PV capacity does not match PCS capacity. Case 7 has an increased PV capacity of 800 kW since there are no PCS systems with a capacity between 500 kW and 800 kW. For each combination, a simulation was carried out based on the test model to review the ROI period. Case 1 is the most general case of a PV-ESS combination. The maximum power demand-per-month for 2015 was checked. July and August were chosen for the highest values of power demanded. Next, the maximum power demand for each day and hour of August was reviewed. The 6 th of August showed the highest demand; therefore, the analysis was completed with the goal of reducing this particular peak in demand. The PV system was set to charge during the time period from 11:00 h to 14:00 h and discharge during 14:00 h to 18:00 h to save cost. For this case, the total investment would be $676,667.5 and profit would be $590,741.47 in ten years, resulting in 10.45 years of ROI period. An economic evaluation was completed with a similar case, but with different capacities of the PV system and ESS. Table 11 presents different properties of the major facilities for each case. After running the simulations for each case, the cases with PV 250 kW, ESS 846 kWh, and PCS 250 kW showed the shortest ROI period of 8.62 years. On the other hand, the case with PV 700 kW, ESS 1,692 kWh, and PCS 800 kW showed an ROI of 12.77, which is the worst result. It is clear that the economic feasibility depends upon the combination of the features of the PV system, ESS, and PCS. In most cases, the engineer chooses a major parameter while using commercially suggested value, and then adjusts the parameter repeatedly until a seemingly optimized result is achieved from an economic evaluation. This would lead to either over and/or underestimated sizes of the capacities. Ideally, the system must be designed by a skilled engineer to display an optimized economic feasibility, which is a challenging task. External factors would affect the system and unlike in the simulation, economic feasibility might not be attained, even if the system was designed perfectly. The conditions for simulation are as follows:  
* Energy generated from the PV system is consumed by the building itself and there is no other grid that is connected. * REC is not considered because the economic evaluation is applied to a general building. * Annual maintenance cost is set to 6% and it is included in total construction cost. * Operating hours of the PV system is set from 11:00 h to 14:00 h, and the energy that is generated from sunrise to 11:00 h is not considered in the economic evaluation. Table 11 shows the detailed information of each case. The initial investment cost of each case consists of the cost of PV, ESS, PCS, PMS, and construction. The ROI of each case differs, because their initial investment cost differs.