## 4 Discussion
In April 2021, it was upgraded to an official international project team (PT600) under the International Electrotechnical Commission (IEC) umbrella. Most formulas, equations, modeling, and measurement methods for photovoltaic devices are based on the assumption that photovoltaic devices are flat, static (with no movement), and illuminated by uniform sunlight. It is a reasonable assumption for typical applications such as rooftop solar panels, but it cannot be applied to vehicle photovoltaic devices. For example, one of the essential equations (Power output) = (Power conversion efficiency) \(\times\) (Area of the device) \(\times\) (Solar irradiance) is not true for the car applications in a physically and mathematically rigorous manner. Such deviation affects the testing procedure. The deviation in PV device performance on vehicles is sometimes called \"unusual behaviors\". Accurate modeling needs to be developed to understand these strange behaviors. On the contrary, such advanced modeling requires advanced math, so not everyone can handle it, and often, it is not transparent when rating the product value. A simple approximation method is needed for rating purposes. Therefore, our standardization activity deals with three layers of testing and rating methodologies (Table 2). We need three layers because reproducible tests can only be secured in testing laboratories. Still, outdoor operation results are inherently different from test results (in principle, the test result is the same as the outdoor operation in flat PV devices after temperature and spectrum corrections). Note that most of the formula and testing methods are based on the assumption that the PV device is flat. Still, we need practical and reproducible testing methods (that are at least applicable by controlled testing laboratories, layer 1). Unlike the flat PV devices (where the outdoor performance will be the same if sufficient corrections like temperature are performed), the behavior of the outdoor operation of curved PV modules is challenging to understand (most operation models are based on the assumption that PV devices are flat). We need a detailed translation procedure for outdoor operations to avoid misleading or unfair recognition. This is why we need layer 2. A thorough and accurate calculation requires a tensor-matrix-vector computation, which not everyone can perform. Moreover, VIPVs frequently work in shaded and partially shaded environments. Therefore, we must provide an approximated but simple and transparent calculation procedure for a fair rating of the VIPV and SEV values (possibly related to carbon offset values). This is why we need layer 3. The geometrical conditions and coordinate system (local coordinate system) can be applied to building-integrated photovoltaics (BIPVs), aircraft PVs, and PV arrays installed on wavy land. Except for reducing the requirements of the geometrical properties, the models and equations and Scheme 1 in Sections 2.2-2.4 cover the entire situation. The differences are summarized as follows. BIPVs: The elevation angles are typically fixed to \(90^{\circ}\), a particular case in the algorithm. PV devices usually do not move, which is also a specific case in the algorithm. The shading (aperture) matrix issue varies with the position of the PV arrays and can be covered by allocating the different matrixes depending on the device's height (typically). Aircraft PVs: The surface model defined in Section 2.4 can be applied to the saddle-curved surfaces that are typical in aircraft wings. As explained in Section 2.5, we also need to consider the hidden surface. PV arrays installed on wavy land: The envelope of the PV arrays is regarded as the curved surface of the PV device, and the mismatching losses are considered to be constrained by the conservation law of the carriers in a series circuit. The aperture matrix can describe the shading effects of the sloped land [71]. This paper showed the validation result in vehicle applications using c-Si solar cells.