## 1 Introduction  
The worldwide interest in the market and technologies related to solar photovoltaic (PV) energy is increasing mostly due the increasing demand for sustainable energy production and energy security, which are aggravated by issues like climate change and environmental problems associated with the development of conventional energy resources [1]. Many countries are making efforts to transform current fossil-oriented energy policies into renewable and sustainable low-carbon policies, which have been promoted by several national and international agreements, such as the 21 st Conference of the Parties (COP21) in Paris and the Green New Deal policy [2]. The forms of solar power applications are becoming more diverse and now include agrivoltaics [3; 4; 5], floating PV [6], minevoltaics [7], generation in remote islands [8], building-integrated PV (BIPV) [9], and rooftop PV [10] systems. Accordingly, solar power projects can be implemented in both national or household and personal scale. Therefore, feasibility studies for solar PV energy projects have also expanded beyond the domain of experts (e.g., scientists, engineers, and policy makers) to the public and personal domains (e.g., energy prosumers and young students). Various solar PV tools used in PV projects have been developed, including desktop software, web-based programs, and smart-device (i.e., smartphones and tablets) mobile applications (apps). Most desktop software have been developed for professionals and their price range of varies greatly, but they are generally not cheap. Conversely, apps for sartphones and tablets (hereafter mentioned simply as smartphones) can be free or have low costs, and they can be used for various purposes in a PV project, including system optimal settings. Their versatility includes rapid processing, mobility, and connectivity between wireless networks and sensors, enabling physical properties of the smartphone and the surrounding environment to be measured. Recently, people have attempted to use portability and sensing technology of smartphones to overcome what was unfeasible with previous single-platform system [11]. Over 4 billion people are using smartphones by 2020 and the number of users is expected to continue to grow [12]. These smartphone apps related to PV energy have been used not only for scientific purposes but also for educational use, such as in new teaching strategies or to introduce cutting edge technology associated with scientific concepts for future energy prosumers. Since more solar PV apps are anticipated to be developed, an overview of solar PV smartphone apps in the perspective of present limitations, and future prospects is required. Several researchers have reviewed solar PV design tools and methods [13; 14; 15; 16; 17]. One of the first comprehensive recent reviews on solar design tools and methods was reported by [PERSON] [13], in which various state-of-the-art software and features were investigated and summarized, focusing on solar potential assessment for BIPV or urban areas. [PERSON] et al. [14] validated and compared the accuracy of the most common PV modeling tools (e.g., SAM, PVWatts, PVsyst, PV*SOL, TRNSYS, Archelios, Polysun, and PVGIS). [PERSON] et al. [15] examined a wide range of methods from simple two-dimensional (2D) analytical models to more sophisticated three-dimensional (3D) representation and numerical analysis coupled with geographic information system (GIS) tools, for modeling solar potential in urban environments. [PERSON] et al. [16] investigated 23 solar PV design and management software and 4 smartphone/tablet applications in terms of catering all geophysical, technical, economic, and environmental factors. [PERSON] et al. [17] reviewed numerous studies on GIS-based tools and approaches for solar radiation mapping, site evaluation, and generation potential. All these reviews tended to focus on desktop- or web-based software and tools (or cover desktop solution only), rather than smartphone apps.