In the past years the Norwegian PV market has grown substantially. The importance of data on how PV systems perform in Norway is therefore also increasing. In this thesis several Norwegian PV systems are analyzed mainly in terms of annual specific yields. The systems are located in western and southeastern Norway and near horizontal, tilted and vertical modules are looked at. Results show pretty similar performance of the near horizontal systems, so-called east/west applications, with 700 kWh/kWp being an approximate average annual specific yield. Vertical systems show large variations in yield due to shading losses and suboptimal orientations. Nonetheless, vertical systems having been placed well show great potential, having annual specific yields on the order of 700 kWh/kWp while producing far better than near horizontal systems in winter months. Not surprisingly systems tilted $30\degree$ or more give the highest yields, slighty above 900 kWh/kWp seeming like a fair approximation for the annual specific yield of such systems in southern Norway. The measured yields have also been compared to estimated yield, giving a rough average deviation of 5\%. Irradiation data from 57 locations in Norway has also been gathered, confirming that the average annual irradiation lies somewhere between 700 and 1000 kWh/m\textsuperscript{2}. The three irradiation models/databases Meteonorm 7.1, PVGIS and NASA SSE have been compared to this data by the means of RMS errors, indicating that Meteonorm and PVGIS perform similarly while the NASA database is less precise. Large local differences are observed though, making some models preferable in certain locations. Simulations performed in PVsyst using average global and diffuse irradiation data from Trondheim, Bergen and Ås show that the increase in yield gotten from tilting a PV system varies not only with latitude but also the amount of diffuse irradiation at each location. Results indicate that the overall relative increase in yield gotten from tilting is the lowest in Bergen and highest in Ås, while the increase from tilting modules $10\degree$ is highest in Trondheim.