This paper describes a new software tool named “VR4PV,” which has been developed for real-time simulation of irradiance for photovoltaic (PV) products in a virtual 3-D environment. This tool offers the possibility for product designers and architects to evaluate the distribution of irradiance on surfaces with an arbitrary geometry that can be covered with PV cells. In addition, the energetic performance can be estimated during the design process of PV products and building integrated PV (BIPV). The software allows for irradiance calculations on multiple arbitrarily oriented surfaces at the same time. It includes shadow simulation for multiple surrounding objects with various shapes and can handle movements of the 3-D objects during the simulation, which might be useful for the design of moving PV-powered products like boats, cars, and portable handhelds. A validation is carried out based on 1-min outdoor measurements of irradiance on two different locations in Italy and in California.