## 4 Conclusions  
In this work, a study of the monthly declination rate of the PR (or \(\Delta\)_PR_%) of a utility-scale photovoltaic installation that operates in a subtropical climate (located on the south-east coast of Tenerife, Canary Islands) during a period of 8 years is presented. Three models were defined. The \"null model\" directly considers the relationship between PR and time, which resulted in a \(\Delta\)_PR_% of 0.0391%/month. This is a very relevant result because, to our knowledge, a monthly decline rate of the PR for PV installations has not been reported in the literature. Moreover, on an annual basis, this figure translates to 0.4692%/year, which is slightly below the degradation rates reported when analyzing individual modules or entire PV systems operating elsewhere, despite having the installation working in considerably harsh conditions, both in terms of irradiation and climatic exposure. This is an interesting result for energy stakeholders interested in sustainable energy investments, since it indicates the suitability of establishing these types of facilities in this kind of environment. Considering that during the construction of the facilities, modules of different types were used, the \"typology model\" studied the relationship between PR and, as covariates, three factors (_Manufacturer_, _Technology_, and _NominalP_) and time. The results indicated that only the \(\Delta\)_PR_% related to _NominalP_ was significant, so that when the nominal power of a type of PV module used is increased by one unit, the variation of the PR of the corresponding PV production unit per elapsed month decreases by 0.000897%. Thus, for instance, a PV production unit that was built with 175 W modules presents, on average, a \(\Delta\)_PR_% 0.2153%/year lower than one that was built with modules of 155 W.  
Taking into account the geographical situation of the facility, its size, and the distribution of the PV production units within it, the \"location model\" considers the relation between PR and, as covariates, two factors (_Edge_ and _LengthSt_) and time. The results indicated that both covariates were significant. Thus, for the _Edge_ one, the variation of the PR of a PV production unit located at the easternmost edge (or border) per elapsed month was, on average, 0.0132% larger than for one not located at such position. Again, on an annual basis, this value translates to 0.1584%/year. The result for the _LenthSt_ covariate indicated that for each PV unit that increases the length of a street, the variation of the PR increases, on average, by 0.00117% per elapsed month. As before, this result adds up; thus, for instance, a street with 10 PV units has, on average, a \(\Delta\)_PR_% of 0.1264%/year higher than a street that has just one unit. These results were related to the exposure to the local conditions, particularly to the prevailing winds, and suggested ways to improve the PR of PV production units, both for those that are already in operation and for those that could be installed in this (or a similar) place in the future. Finally, the obtained results were translated to economic terms, in order to quantify their impact so that they could be useful for those stakeholders interested in investing in this type of sustainable energy. A future line of research will address the effects of the curtailment orders on the energy produced, issued by REE. In this line, we will extend the present study beyond 2019 in terms of energy not produced by renewable sources, as well as their economic and environmental costs, taking into account not only the results presented in this article but the changes in the energy legislation in the Canary Islands. **Author Contributions:** Conceptualization, C.M., R.D.-G., B.G.-D., S.G.-P., L.O. and E.L.; methodology, C.M., R.D.-G., B.G.-D., S.G.-P., L.O. and E.L.; software, R.D.-G.; validation, C.M., R.D.-G., B.G.-D., S.G.-P., L.O. and E.L.; formal analysis, C.M. and R.D.-G.; investigation, C.M., R.D.-G., B.G.-D., S.G.-P., L.O. and E.L.; resources, C.M., L.O.