## 4 Discussion  
To be able to properly design a PV system to meet the energy needs of transforming the traditional system to a more sustainable system, several scenarios were proposed for the PV system design. The scenarios were based on what areas to use, and analyzed based on energy yield, using actual shading data that were acquired from the physical location. A grading system was proposed, based on these results. The equation proposed uses weights that represent project priorities. In this case study, it was important to produce the required energy, with minimal losses, and a small overhead to make use of the connecting net metering scheme. If a project has different priorities, such as limitless energy injection profit, then the priorities and weight can be changed to reflect that specific scenario. The goal is to use this type of information to provide the designer or decision maker with additional information for better design decisions.  
Scenario 3 has the highest score and meets capacity requirements, with lower losses and a higher specific yield than scenarios 1, 2, 5, and 6. This scenario proposes installing PV panels with an overall capacity of 20,017.00 kWp in order to generate 34,696,000.00 kWh per year. As the generated energy is slightly above the requirement, it provides a safety margin. This is important, as the design is based on a simulation; and as the system will be connected using a net-metering scheme, excess energy is forfeit and energy deficit is billed.  
Deploying this sustainable upgrade will result in eliminating the release of 27,204 metric tons of CO\({}_{2}\) a year. As the project has an expected lifetime of 15 years; this results in 408,060 metric tons of CO\({}_{2}\) emission reduction. Regardless of what scenario is chosen, as long as all energy required from the upgrade is supplied through a renewable energy sources, then all emissions are eliminated.  
\begin{table}
\begin{tabular}{|c|c|c|c|} \hline CAPEX & 15 Years & Baseline Buses & 86,765,290 USD \\ (present worth) & & PV powered Electric Buses & 141,892,633 USD \\ \hline \multicolumn{3}{|c|}{Incremental Capital Cost} & 55,127,343 USD \\ \hline OPEX & \multirow{2}{*}{15 Years} & Baseline Buses & 105,862,835 USD \\ \cline{3-4} (present worth) & & PV powered Electric Buses & 4,283,408 USD \\ \hline \multicolumn{3}{|c|}{Savings in Operational Cost} & 101,579,427 USD \\ \hline \multicolumn{3}{|c|}{Payback Period (Years)} & 7.1 \\ \hline \end{tabular}
\end{table}
Table 10: Capital expenditure (CAPEX) and operational expenditure (OPEX) for the baseline and green streets model in net present value.  
Figure 8: Operational costs for PV electric bus model.  
After converting the capital investments for the baseline buses and PV-supplied electric buses to net present value (NPV), and using the same approach for OPEX, it was found that the NPV for the incremental investment is 55,127,343 USD, while savings in operational costs are 101,579,427 USD over 15 years. With those results, the payback period is around seven years. For such an important project and the huge impact on the environment and society, seven years for returning the initial investment is very good, bearing in mind the calculations were based on a 15 year project lifetime. Much of the installed equipment will last for much longer.  
The total cost of ownership (TCO) for the baseline buses and proposed model is another valid comparison. It was found that the TCO for the business-as-usual scenario is much higher than the TCO of the PV electric bus model by about 32%. TCO for business-as-usual scenario in NPV is 192,628,125 USD over 15 years, while TCO for PV electric bus model in NPV is 146,176,041 USD over the same time period.