PV windows made from c-Si and a-Si ZM447439 are considered to be a mature and well developed technology. Therefore, many studies and various applications have been reported in the literature. On the other hand, emerging PV technologies are still under development and building applications are limited (DSSC) or non-existent (OPV). Apart from electricity generation, PV windows also affect the building?s energy performance by substituting its exterior finishing materials with the creation of a pleasant indoor environment . The transparency of PV window is one of the most important issues in their development, since it is connected to the optical and thermal properties of the window and the building in general. By comparing different PV technologies in a face to face methodology (Fig. 10a)  under the same boundary and climatic conditions, the best PV technology can be selected. In similar way, the real energy contribution of the PV system on the overall energy balance of the building can be calculated. In the same methodology, potential problems can also be accessed and key information can be revealed.