The first zero-energy office building in Singapore uses building integrated photovoltaics to meet its energy target. The main electricity generator is the 142.5 kW(p) grid-connected BIPV system on the roof of the building. This paper presents the first performance assessment of the BIPV system over 18 months of operation following the guidelines of the IEC standard 61724 for measurement, data exchange and analysis. The performance analysis shows a good overall performance ratio of 0.81. The average array yield is 3.86 h/d and - after subtracting the capture and system losses of 0.58 h/d and 0.16 h/d, respectively - the resulting final yield is 3.12 h/d averaged over all arrays. The system and array efficiencies are 11.2% and 11.8%, respectively, compared to the nameplate PV module efficiency of 13.7%. The overall inverter efficiency is 94.8%. All the results are based on irradiance measurements with calibrated pyranometers. In addition, a classification of daily irradiance is presented, dividing the days into overcast, intermediate and clear days with each high, medium and low temporal change of irradiance levels. Results show that the performance ratio is lowest for clear days with high irradiance fluctuations, due to higher capture and system losses. Despite relatively higher losses, the absolute final yields are higher for clear days. Detailed analyses with respect to the impact of shading, orientation/tilt, and PV module temperature are also presented for selected arrays. (C) 2012 Elsevier Ltd. All rights reserved.