The loss mechanisms for large grid-connected amorphous double-junction photovoltaic (PV) system application under high latitude climate conditions were systematically analysied. A two years analysis of data from the 10 kWp façade double-junction amorphous system, which forms part of the 60 kWp grid-connected PV system that was installed at the IKEA main office building in Älmhult (56.6?Sweden, in November 1997, forms the basis of this investigation. The analysis found that the system performance ratio (PR) relative to Standard Test Conditions (STC) specifications in 1998 and 1999 were 47% and 41%, respectively, with monthly values as low as 23% in December 1998. The investigation’s aim was to ascertain the numerical contribution made by each of the different loss mechanism, as they relate to PV system operations, to the performance of the system. The effect on the system’s performance by the following four losses were investigated: a) air mass (AM) effect, PRAM b) irradiance and incident angle effect, PRIRR&INC c) inverter effect, PRINV and d) mismatch and others effect, PRMIO. The numerical analysis employed in this study showed that the performance of the double-junction amorphus façade system due to the different loss mechanism (PRX) was seasonal dependent. Where PRAM and PRINV decrease by 28% and 10%, respectively, from June to December and PRIRR&INC and PRMIO increase by 10% and 6%, respectively, over the same period in 1999.