## 3 Overview and Taxonomy of Solar PV Monitoring System  
The architecture of the solar PV monitoring system can be divided into three levels, namely, the data acquisition level, the data processing level, and the data display and storage level, as illustrated in Figure 3[27]. In the data acquisition level, the data are collected from the various sensors such as voltage, current, temperature, humidity, irradiance, etc., and are then sent to the next stage using either the wired or the wireless system. In the next level, the data are stored temporarily in auxiliary devices such data loggers, processed, and sent to the last stage for the display of the results. In the last level, the data are received by the workstation and then the required actions are taken by the system to be configured accordingly. These data can be accessed from anywhere at any time via the internet. A comprehensive review of solar PV monitoring system-based data processing modules and data transmission protocols has been carried out. The data transmission modules allow the acquisition, control, and management of connected devices and networks in a real-time environment. Furthermore, they act as middleware that connects with different devices through cloud computing technology. The solar-based monitoring system is comprised of a four-layer structure which consists of sensors (electrical and environmental) for gathering data such as panel voltage, current, temperature, humidity (small scale) etc., a network layer for the transmission of the data by utilizing various transmission protocols  
Figure 3: The basic architecture of a solar PV monitoring system. such as ZigBee, Wi-Fi, Bluetooth, LoRa etc., a data processing layer for processing necessary data through various data processing modules such as Raspberry Pi and Arduino, and lastly, an application layer which acts as an interface between end devices and the network as depicted in Figure 4. Further, [PERSON] et al. [29] explored the functionality of the various layers such as the sensor, transmission, data processing, and application layers. The authors revealed that the change in the execution of various layers is based on applications such as small scale or large-scale monitoring, and software-based or cloud-based monitoring. The classification of various electrical and environmental parameters based on large-scale and small-scale solar PV systems is presented in Table 1. Platforms such as BeagleBone, Arduino, Raspberry Pi, PLC, and the microcontroller chip have been reviewed in relation to the application of data transmission protocols, the parameters measured, the software used, the monitoring platform, and the related results. In addition, the monitoring system consists of several network protocols for communication to ensure the protection of data during transmission between connected devices [30] Furthermore, issues relating to low range, low computational speed, low storage memory etc., motivates the requirement of the communication protocol for the monitoring system [31]. In this regard, communication protocols utilizing various wireless communication modules such as ZigBee, Wi-Fi (ESP8266 module), Bluetooth, GSM, and the LoRa module have been reviewed for monitoring solar PV systems. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Solar PV System**} & \multicolumn{2}{c}{**Parameters**} \\ \cline{2-3}  & **Environmental** & **Electrical** \\ \hline \multirow{4}{*}{Large scale} & Irradiance & Array output voltage \\  & Array Temperature & Array output current \\  & Speed of wind & Grid voltage \\  & Humidity & Current to and from grid \\  & Air pressure & Grid impedance \\ \hline \multirow{4}{*}{Small scale} & Irradiance & Panel output voltage \\  & Panel Temperature & Inverter output voltage \\ \cline{1-1}  & Humidity & Inverter output current \\ \cline{1-1}  & & Load output voltage \\ \cline{1-1}  & & Load output current \\ \hline \hline \end{tabular}
\end{table}