## 5 Conclusions  
We presented a feasibility study on the retrofitting of existing buildings in compliance with Thailand's building energy code. Five aspects of the building were evaluated using the BEC software: the building envelope, lighting, air conditioning, renewable energy and total energy consumption. The assessment results for the base case reveal that the building does not meet the standard in the envelope section and air conditioning system, while the lighting system and total energy consumption are still within the standard allowance. However, the values are close to the standard limit. Therefore, they can stop fulfilling the standard in the future in cases of aging equipment, materials and revision of building energy codes.  
The retrofitting of the case study was performed and assessed in compliance with the building energy code using BEC software. According to the results, the envelope was improved by installing an insulator in the wall and roof, and the window was changed into high-energy-efficiency glass, which allowed the building to fulfil the envelope section of the building code. The lighting system was also retrofitted by substituting fluorescent luminaires with high-energy efficiency LED luminaires, which can provide a significant reduction in energy consumption and maintain the same amount of illuminance on the working plane. Concerning the air conditioning system, the fixed-speed air conditioning units were replaced with variable-speed (inverter) air conditioning units. The replacement unit has a double SEER value compared to the base-case unit, which results in the same cooling capacity and decreased energy consumption. In the retrofitted case, a PV system was installed, and the metal sheet roof was removed such that the generated power significantly reduces the total energy consumption with respect to the reference and base-case building. Thus, the retrofitted building can reduce the energy consumption by approximately 48% compared to the base case. The result has shown that a reduction of energy consumption, by completely retrofitting the building to be of an energy efficiency similar to the case study building, has the potential for an energy saving estimate of around 3000 GWh.  
From an economic perspective, the economic indicators show that retrofitted existing buildings have potential in terms of feasibility and investment attractiveness, with an IRR value of 19.32% and a DPP of approximately 4.36 years. From both energy and economic perspectives, retrofitted existing buildings in compliance with Thailand's building energy code can benefit project owners with a fast payback and the possibility of generating profit in the long term. In addition, electric utilities can also benefit from a reduction in investment in utility upgrading and provide energy efficiency for electrical networks. However, there must be policy, support and incentive from both private and government entities to initiate the energy savings measured by retrofitted existing buildings in order to achieve energy saving targets.  
Conceptualization, S.A. and A.N.; methodology, S.B.; software, T.P.; validation, S.A., A.N. and S.B.; formal analysis, S.A.; investigation, S.A.; resources, A.N.; data curation, T.P.; writing--original draft preparation, S.A.; writing--review and editing, A.N.; visualization, S.A.; supervision, S.B.; project administration, S.B.; funding acquisition, A.N. All authors have read and agreed to the published version of the manuscript.  
This research was funded by King Mongkut's Institute of Technology Ladkrabang, grant number KDS 2019/005.  
Not applicable.  
Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
This work was supported by King Mongkut's Institute of Technology Ladkrabang (No. KDS 2019/005), Thailand.  
The authors declare no conflict of interest.