The photovoltaic system of 150WP in Falun is on the sloped roof which is 14 degree with east and west orientations. A calibration of the radiation sensors has been done in order to correct the voltage drop caused by the extended cable length. The radiation sensors are calibrated by comparing the measured irradiance of the PV system with the reference irradiance which is measured by a portable data logger system. The annual performance of the system is evaluated over one year from April 08 to March 09 according to IEC 61724. Two other PV systems are compared with Falun system based on the annual performance of each system. The total system energy production of one year is about 96 MWh and the performance ratio PR is 0.817.