## 4 Classification and Features of Smartphone Apps for Solar PV Energy
### PV System Optimal Settings  
The second function of solar PV apps is information and design of PV module (optimal setting). Table 4 lists the corresponding smartphone apps, including their developer/company, platform, price, and main functions. Many of these apps calculate the optimal tilt (inclination or slope) and/or orientation (azimuth or aspect) of PV modules, and analyze the separation distance between PV modules according to their geographic location. Moreover, many of them propose (or provide information on) a combination of solar modules, inverters, batteries, and other elements of PV systems according to the electric power consumption (load). Their prices ranged within 0-9.99 USD. The tilt angle of the PV module is the angle in degrees from the horizontal line, and the orientation refers to azimuth setting. The PV module collects solar radiation directly from the sun and the sky, and sunlight reflected off the ground or areas surrounding the PV module. PV modules should be positioned in a direction and tilt angle that maximize their exposure to direct sunlight to optimize their collection efficiency. Therefore, tilt and orientation factors (TOF) are crucial in PV system optimal settings because the output of the PV system can be increased by optimizing them based on geographic location. The module collects solar radiation most efficiently when the sun's rays are perpendicular to its surface. However, the angle of the sun varies throughout the year. Therefore, the optimal tilt angle for a PV panel in the winter will differ from that in the summer. This angle will also vary according to latitude. Many apps can calculate the optimal tilt and/or orientation of PV module based on time and place. A few apps only calculate optimal tilt for annual [31], winter [32], seasonal [33], monthly [34], and daily [35], whereas one app suggests calibration value to both optimal tilt and orientation of PV modules by placing a smartphone on the solar panel bracket [33]. These apps provide the optimum tilt angle or orientation of PV modules based on the geographical coordinate using GPS data or when the user inputs latitude and longitude manually. Numerous apps also recommend the type, size, and number of PV modules, inverters, batteries, and chargers according to a given power consumption (load) [36; 37; 38; 39; 40; 41; 42; 43]. The overall features and principles of the apps are as follows. They first allow the input of individual data for each power appliance or total energy consumption, and the PV module specifications in a simple design. Second, the user sets options associated with PV power output such as weather conditions, area of installation, separation distance, and power losses. In this process, a few apps allow the user to manually input the amount of sunlight (e.g., average daily sunshine hour or average daily global horizontal irradiance (GHI)) [37; 38; 39; 41; 42; 43]. Then, a link to the National Renewable Energy Laboratory (NREL) website are provided to retrieve and apply local solar radiation via a map with legend in a specific app [40], as seen in Figure 2. Thus, most of these apps have limitations that cannot accurately reflect weather information based on exact location. Third, this app calculates the number of required PV modules and batteries based on user's power consumption loads and PV module specifications. The Solar Calculator app [44] is a tool for homeowners to define the required size of a roof PV system. This app provides information and compares various PV modules from different manufacturers, and it calculates the required number of modules according to the size and by considering the power consumption and degree of exposure in the roof. Another app simultaneously calculates the number of PV modules and batteries [45]. The PV Module Calculator app [46] provides a simple design, and it calculates various characteristics related with PV systems, such as installation area, degradation separation, number, price, weight of PV module, daily power generation time, and distance between PV modules.