## 9 Conclusions  
The size and rated power of USPVPs have been increasing exponentially in the last decade, increasing the need for optimisation and automation of maintenance processes. Currently, one of the  
Figure 13: Annual prorated costs resulting from the EL maintenance application for installed MWp for the selected use case. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Feature** & **Illuminated IRT** & **Dark IRT** & **Off-site EL** & **On-site EL** \\ \hline
**Implementation test cost** & + & + & ++++ & + \\
**Plant production losses** & - & - & +++ & - \\
**Results accuracy** & ++ & + & +++ & +++ \\
**Time per test** & + & + & +++ & ++ \\ \hline \hline -: null, +: low, ++: moderate, ++: high, ++++: very high. & & & \\ \end{tabular}
\end{table} 
Table 3: Features and costs comparison between state of the art and proposed USPVP image processing methods. most extended practices in the maintenance of PV panels is the analysis of IRT images. However, as has been explained through the text, IRT technique has some limitations in the detection of defects in PV panels. One of the most promising techniques to complement the results of IRT tests is the study of EL images. Unfortunately, EL procedures involves dismantling the modules or to use external power supplies and, therefore, they have a high cost and require qualified staff and special equipment. In this context, the paper has presented a novel USPVP maintenance technique based on on-site EL image acquisition by means of the bidirectional inverters usually installed in USPVPs. The proposed procedure retains the high capability to characterise defects inherent in EL technique, achieving a high automatisation level and, as a consequence, a time and cost reduction. The behaviour of the system has been calculated by simulation, and has been validated with experimental results in a small-scale PV plant successfully, afterwards. The results confirm the technical feasibility of performing this technique on a USPVP, without the need to uninstall and reinstall the panels for their evaluation. Consequently, on the one hand, the presented technique allows a full evaluation of the plant at a low operational cost, due to the low energy consumption performing the test. Moreover, on the other hand, the technique could be applied to both new and existing plants at a low installation cost, due to the fact that only minor hardware and software changes are needed. In particular, hardware costs of the installation for a real scenario have been calculated, representing a neglecting increment in the plant installation cost per megawatt, and the cost of software modifications is almost zero too, because it is very simple, with low computational requirements, and easy to implement algorithm in any commercial inverter control unit. In conclusion, the paper has provided a new cheap and valuable solution to improve the maintenance of USPVPs; a method that could be used as an alternative or complementary with IRT technique. Indeed, some resources such as the unmanned vehicle could be shared between the two methods, reducing the overall installation cost for both of them. Following the evaluation in the pilot-site with successful results, future work will test the solution in a real USPVP, in order to evaluate the solution in an industrial environment. Conceptualization, J.B.-F., J.M.-C.-A. and L.H.-C.; methodology, [PERSON]-C.-A., J.F.S.-O. and L.H.-C.; software, [PERSON]-F., [PERSON]-C.-A., validation, [PERSON]-F., [PERSON]-C.-A., L.H.-C. and V.A.-G.; formal analysis, [PERSON]-F., [PERSON]-C.-A., [PERSON]-C. and J.F.S.-O. ; investigation, [PERSON]-F., [PERSON]-C.-A., L.H.-C. and O.M.-S.; resources, [PERSON]-F., [PERSON]-C.-A., V.A.-G., [PERSON]-F., J.I.M.-A.