## 4 Retrofitted Results
### Economic Results  
An economic result can be evaluated by calculating investment cost and electricity cost reduction. The electricity cost in terms of time of use tariff (TOU tariff) for the case-study building is 4.3297 baht per kWh or 0.14 USD per kWh [29]. The interest rate used to calculate the present value in this study was 1.75% according to the forecast of the Bank of Thailand [30]. The maintenance factor for the building envelope, which includes the deterioration rate of the building material, was set to approximately 0.8% per year in this study. The results in terms of net energy saving from the BEC software in the case of a retrofitted building were used to calculate the energy cost savings in terms of electricity fee, as summarized in Table 21. The annual cash flow from the retrofitting project for 10 years is comprehensively shown in Table 22, and the results in terms of economic indicators are shown in Table 23.  
The tables show that the initial costs are high. However, the income in terms of energy cost savings can compensate for the investment cost each year. The amount of energy saving is decreasing each year from the aging of equipment and some maintenance requirements, which are included in the maintaining factor. Concerning economic indicators, the energy savings from retrofitted existing buildings can achieve a DPP within a period of 4.36 years and has an IRR value of 19.23%, which can quickly pay back an investment and constitutes  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **NPV (USD)** & **DPP (years)** & **IRR (\%)** \\ \hline Retrofitted case & 340,179.32 & 4.36 & 19.32 \\ \hline \hline \end{tabular}
\end{table}
Table 23: Economic results for the retrofitted case.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Quantity** & **Price Per Unit (USD)** & **Installation (USD)** & **Total** \\ \hline Monocrystalline panel 370 W & 220 & 125.00 & - & 27,5000.00 \\ Inverter 25 kW & 3 & 6500.00 & - & 19,500.00 \\ Structure and miscellaneous electric equipment & - & - & 10,000.00 & 10,000.00 \\ Installation cost & - & - & 26,666.67 & 26,666.67 \\ Total & & & 83,666.67 \\ \hline \hline \end{tabular}
\end{table}
Table 20: Equipment and installation cost of renewable energy system.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & **Initial Cost** & **Maintenance** & **Energy Unit Saving** & **Energy Cost** & **Cash Flow** & **Future Value** & **Balance** \\  & **Factor** & **(kWh/year)** & **Saving (USD)** & **(USD)** & **(USD)** & **(USD)** & **(USD)** \\ \hline
0 & 319,081.00 & - & 547,616.77 & - & (319,081.00) & (319,081.00) & (319,081.00) \\
1 & & 0.80 & 543,235.83 & 76,053.02 & 76,053.02 & 74,561.78 & (244,519.22) \\
2 & - & 1.60 & 538,854.90 & 75,439.69 & 75,439.69 & 72,510.27 & (172,008.95) \\
3 & - & 2.40 & 534,473.96 & 74,826.35 & 74,826.35 & 70,510.55 & (101,498.41) \\
4 & - & 3.20 & 530,093.03 & 74,213.02 & 74,213.02 & 68,561.36 & (32,937.04) \\
5 & - & 4.00 & 525,712.09 & 73,599.69 & 73,599.69 & 66,661.51 & 33,724.46 \\
6 & - & 4.80 & 521,331.16 & 72,986.36 & 72,986.36 & 64,809.80 & 98,534.27 \\
7 & - & 5.60 & 516,950.23 & 72,373.03 & 72,373.03 & 63,005.08 & 161,539.35 \\
8 & - & 6.40 & 512,569.29 & 71,759.70 & 71,759.70 & 61,246.21 & 222,785.56 \\
9 & - & 7.20 & 508,188.36 & 71,146.37 & 71,146.37 & 59,532.10 & 282,317.66 \\
10 & - & 8.00 & 503,807.42 & 70,533.04 & 70,533.04 & 57,861.66 & 340,179.32 \\ \hline \hline \end{tabular}
\end{table}
Table 21: Energy cost saving after retrofitting a building.  
a very attractive investment indicator. An assessment based on economic indicators reveals that the retrofitted project is feasible and has the potential to return an investment to the project's owner. This means a lower price resulting from high-efficiency materials and equipment.