## 2 Smartphone Technology and Built-in Sensors  
Before reviewing the apps, the main features and functions of smartphones should be understood. A smartphone is a device that combines communication and sensing functions in a mobile computer. Therefore, it can be used to create, analyze, visualize, and manage various data using built-in software. It is continuously connected to a communication network, and it can perform field measurements when combined with internal or external sensors. With recent developments, smartphones can perform desktop computer-level computations, and the range of smartphone apps has significantly expanded, including those that desktop applications cannot perform due to the use of smart sensor technologies. Moreover, smartphones are an intuitive and easy-to-use device, thereby having cost-effective advantages when designing apps to substitute for existing approaches or software [11]. Numerous state-of-the-art smartphones have a variety of built-in sensors that can detect physical properties associated with three axes (x, y, and z) and are capable of measuring local electromagnetic fields and acceleration [18]. Table 1 summarizes various sensors that can be built into smartphones and their functionalities and expected roles in solar PV apps. These sensors can enable users to acquire physical property information and environmental factors for solar PV systems. The multifaceted sensor applications enable the measurement and visualization of data, which is seldom found in conventional desktop/web-based software. The global positioning system (GPS) is a fundamental built-in sensor that can be used to measure and visualize on-site data associated with solar PV within a smartphone because most information is closely related to geographical location. This sensor calculates the geographical coordinate (latitude and longitude) of the device necessary to obtain additional on-site information for solar design purposes. For instance, latitude and address can be used as input data to assess PV potential. The optical image sensor can be used to acquire optical images as photos or videos. Furthermore, this sensor can visualize the position and trajectory of the sun over time. A particular advantage is that the image sensor can be used to easily check the position of the sun not only outdoors but also indoors when combined with accelerometer and magnetometer that measure the tilt and direction of the smartphone, respectively. The accelerometer and magnetometer can be used to measure the tilt or orientation, respectively, of the PV module or roof. The detailed principle of measuring tile and orientation can be found in [PERSON] et al. [18]. Moreover, environmental sensors (i.e., barometer, ambient temperature, and humidity sensor) are capable of measuring on-site weather conditions with the aid of GPS data. These measured data can be used as input parameters to predict PV power generation. External sensors that can be connected or attached to smartphones are being actively developed. Examples include solar radiation sensors (pyranometer), thermal infrared sensors, and radioactive detectors [19]. In the past, these external sensors were used for specific equipment or devices, not smartphones. However, with the development of the Internet of Things, opportunistic factors in which various smart sensors are developed have led to this change. Nevertheless, most of these external sensors are still developed