Since solar panels mounted on building facades are expected to come into wide use in urban areas, accurate estimation of photovoltaic potential of building facades is necessary for urban energy management planning. Accordingly, we have planned to develop a system to estimate photovoltaic potential of building surfaces using a 3D city model. The paper reports the first step of the development, which is the development of a method to estimate solar irradiance of urban buildings. The most significant feature of the method is that the method adopts the idea that “whether the sun can see us or not” indicates “whether we have sunshine or not” for calculation of solar irradiance of a point on a building surface. Utilization of projection images viewed from the sun created by using computer graphics (CG) techniques such as the Z-buffer algorithm makes our system much less computation time than most of existing systems using a hemispherical viewsheds or that of ray tracing. Results of an experiment conducted in Yokohama of Japan demonstrate that the method would be able to estimate solar irradiance on not only roofs and but also facades of urban buildings using LOD2 building data in Japan accurately enough.
