## 5 Methodological Review of Existing Literature
Again, two types of research can be identified: studies investigating EVs as a solution to reduce emissions by themselves (i.e., without adding RES), and those directly focused on other aspects while assuming a future renewable mix. **SG** oscillates between 0% and 25.00% (\(\mu_{SG}=11.58\%\) and \(\sigma_{SG}=8.19\%\)). This category is closely related to telecommunications and the Internet of Things (IoT) systems, and frequently involves the management of smart grids; therefore, the presence of high EV penetration shows important dependency on the GI category. Depending on the topics researched in each year's reviews, authors may not specifically indicate this; nevertheless, any grid impact would be impossible without smart grids. **WP** oscillates between 0% and 16.67% (\(\mu_{WP}=7.92\%\) and \(\sigma_{WP}=5.96\%\)). This essential category shows a two-way technical dependency between EV and wind power, with high penetration of both being possible. **SP** oscillates between 0% and 25.00% (\(\mu_{EV}=13.55\%\) and \(\sigma_{EV}=7.17\%\)). This is an essential category for enabling high EV and solar power penetration. To rank the categories and estimate their relative importance and consistency over time, we defined the following two key performance indicators (KPIs). \[KPI_{1i}=\mu_{i} \tag{1}\]  
\[KPI_{2i}=\frac{\sigma_{i}}{\mu_{i}} \tag{2}\]  
In both equations, \(i\) is the corresponding category indicated in Tables 5 and 6. We ordered the previous categories based on these KPIs, with a higher \(KPI_{1}\) value indicating greater relative presence on the part of a category and a lower the \(KPI_{2}\) value indicating greater consistency and stability over time. In Tables 5 and 6, the first places (GI, T, and S) indicate the importance and temporal consistency for the subject and categories of study. Next, SP and SG have similar importance, with their order reversed by only one position based on each KPI. The next place corresponds to the WP category. Finally, the last positions are for C&S and E categories, which had the least impact on the studies. Figure 15 presents the same type of analysis for the set of subcategories belonging to S, incorporating years and average frequency. Table 7 orders the \(S\) subcategories from greatest to least interest and consistency over time according to the KPIs described above. It can be concluded that the subcategory with the greatest research importance is that of optimization algorithms, which are the most interesting tool for researchers. Figure 16 shows a similar analysis for the T subcategories. Using the previous method, Table 8 shows the subcategories ordered from most to least important. At a technological level, the most widely studied subcategories are 'V2 GHV or (VGI) or (G2V)', with high research relevance. Well behind, the second subcategory of 'Battery technology, remanufacturing, and recycling' also accumulates a large amount of research in the study period. Solving technological problems related to both topics is essential for massive integration of EVs at the general level. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Subcategory** & \(\text{KPI}_{\text{1I}}\) [\%] & \(\text{KPI}_{\text{2I}}\) \\ \hline Optimization algorithms & 43.94 & 0.47 \\ \hline PHEV/EV penetration and electric system capacity & 35.00 & 0.45 \\ \hline Long term & 10.91 & 1.21 \\ \hline Short term & 10.15 & 1.61 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: _KPI_ analysis of \(S\) subcategories. Figure 15: Temporal evolution of Simulations subcategories from 2012 to 2022. In the case of GI (Figure 17), using the previous method, Table 9 shows the subcategories ordered from most to least important. Between the second and third rows of Table 9, more weight was selected for transformer congestion and line deterioration in distribution networks versus stability of power systems/grid, as the relative variation of \(KPI_{2}\) (4.00%) was greater than that of \(KPI_{1}\) (3.84%). Due to these small differences, both subcategories are considered equivalent in importance.