This paper has the objective of developing a strategy based on the preventive maintenance policy with minimal repairs in the form of mathematical models applied to solar panels interconnected in series. Our approach consist in determining the optimal number of solar panels to replace with the optimal time corresponding to the number of solar panels changed in order to minimize the total maintenance cost. In this perspective, we consider that the replacement of solar panels improves the reliability of the system, however the minimal repairs does not affect the reliability of the system in a finite planning horizon. A numerical example is presented to illustrate the contribution of the proposed modelling approach and to discuss the different results that are considered.