The integration of renewables into power systems has led to multiple studies and analysis in terms of grid-power quality, reliability, and/or feasibility. Among different resources to be considered as alternative energy systems, wind and solar emerge as the most mature technologies. With regard to photovoltaic (PV) installations, monitoring problems requires detailed analysis, since solar-radiation fluctuations, soiling on solar panels, or deficiency of PV-panel performance can involve unexpected power-output oscillations and, subsequently, undesirable power-generation oscillations. Under this framework, this paper describes and assesses a wireless low-cost PV-module monitoring system based on open-source solutions. Our proposal allows us to monitor installations at the PV-module level, giving detailed information regarding PV power-plant performance. The proposed monitoring system is based on the IEC-61724 standard requirements, as a flexible and ad hoc solution with relevant connectivity options. Meteorological and electrical data are collected from the developed nodes and available for subsequent analysis. Detailed information of the solution, as well as extensive results collected in Spanish PV power plants connected to the grid, are also included in the paper. Energies 2018, 11, 3051 2 of 20Best et al. [5] affirms that solar and wind resources are on the upper rungs of the energy ladder, their integration being predominant in higher-income countries. Recent contributions, focused on future electricity systems, also assume that renewable assets can be considered a mix between wind-and solar-power generation [6]. In [7], and apart from wind and PV solar deployment, a slight increase has been estimated during the past few years for other renewable energy sources for electricity. In addition, Ellabban et al. [8] establishes a direct connection between technological maturity and economic barriers, mainly when the cost of a technology is above the cost of other competing alternatives. Therefore, wind and PV solar resources are currently the most integrated renewable energy sources into power systems, excluding hydropower-generation units. Nevertheless, there are no large-area power systems in the world where wind and solar power generate more than 50% of electricity [9].The integration of wind and PV power plants into power systems involves certain technical problems, mainly focused on reliability, power quality, and stability [10]. The intermittent nature of such sources may increase grid stress, mainly due to undesirable oscillations on the supply side [11] that may affect voltage regulation of transmission systems [12]. At a low/medium-voltage level, different reactive power control strategies have also been proposed to facilitate renewables integration [13], including active power curtailment [14]. As an additional solution to guarantee a balance between generation and demand, with reduced capacity and costs of operating reserves [15], short-term forecasting of PV production emerges as a relevant topic of inte...