## 1 Introduction
Hence, it is advisable to include environmental data at a suitable resolution in order to simulate the output power of a PV cell. If environmental data at a lower sample rate are obtained, the data could lead to false estimates [24]. Generally speaking, the available area and surface on top of an electric vehicle for the deployment and integration of PV cells is limited [18]. Areas other than the roof of the vehicle, such as the side, the rear and the front, receive considerably less irradiation [25]. In addition, the circumstances, particularly the rate of change of irradiation, differ significantly between driving in urban areas and leaving the vehicle parked [18; 25]. As part of its research work, Toyota recently presented a solar-powered version of the Prius. Integrated into the vehicle's hood, roof and rear hatch were 1188 PV cells manufactured by Sharp, achieving a conversion efficiency of more than 34%. The number of PV cells could be increased further if other areas of the Prius were also used [26]. However, as the Prius is a plug-in HEV, the capability of storing the obtained PV energy (e.g., in the vehicle's high-voltage battery) can be limited [27; 28; 29]. In this paper, we summarize and categorize our previous research and other related research investigating the environmental impact on the degree of efficiency of PV installations integrated in electric vehicles today and in the future. The focus is on potential ways to maximize and optimize the ERE of plug-in HEVs, BEVs and CAVs. Hence, we present tools and methods that allow us to address the given circumstances (e.g., curved surfaces, parking and driving conditions) and ambient conditions involved in PV installation in electric vehicles. In this paper, the goal is to define the main impacts on the degree of efficiency of PV-powered electric vehicles and to emphasize the importance of understanding environmental conditions that affect PV cells output behavior and performance. Hence, the main contributions of this work are as follows:  
1. A PV simulation model is proposed which allows us to take into consideration the different alignment of PV cells on top of curved surfaces, such as the vehicle's roof, and the vehicle's cardinal orientation towards the sun. In contrast to other models in the available literature, the proposed model in this work allows us to estimate and predict accurately the potential energy of PV-powered electric vehicle. 2. The impact of environmental conditions on PV-powered electric vehicles was studied. While in parking conditions, the circumstances are comparable to PV-powered charging stations, in driving conditions, the available irradiation is changing quickly and rapidly. Example data are presented for PV-powered vehicles in both parking and driving conditions. 3. The challenges to optimize and maximize the degree of efficiency of PV-powered electric vehicles are elaborated. On the example of maximum power point tracking (MPPT) in driving conditions, in comparison to vehicles in parking conditions, the potential reduction in system efficiency is analyzed. A possible solution is proposed to control the operating voltage of PV cells on top of curved surfaces in an effective way. This paper is organized as follows: Section 2 describes the characteristic output behavior of a PV cell. Section 3 presents a suitable simulation model for PV installation embedded in curved surface shapes. Section 4 presents the environmental conditions for PV-powered vehicles in both parking and driving conditions. Section 5 analyzes and discusses the potential degree of efficiency of photovoltaics integrated in electric vehicles. Finally, the conclusion and future works are given in Section 6.