Building Integrated Photovoltaics (BIPV) constitute the way to reach Nearly Zero Energy Buildings and even zero energy districts (NZED). BIPV surfaces can operate on roofs and facades and their efficiency and productivity are related to orientation, shading, reflections from surrounding surfaces. The novelty of the present investigation relies on a chained model also able to account for the Urban Heat Island conditions. The building performance are analysed as EnergyPlus simulations, considering a multi thermal zone reference building located inside a district of similar characteristics. The PV power generation, from hourly to yearly values, is calculated accounting for PV module temperature, irradiance intensity and solar incident angles, by further developing the well known Sandia model. The whole model, applied to a particular city (41.9 degrees N, 12.5 degrees E), shows how the progressive increase of vertical PV surfaces on both the reference and surrounding buildings yields to a reduction of the energy production per PV unit area. The yearly NZED requirements is reached, in the present case, harvesting solar energy on 60% of rooftops and on 60% of the total area of the facades, with a 11% decrease in energy production per PV unit area due to "darkening" effects induced by PV surrounding buildings. (C) 2020 Elsevier Ltd. All rights reserved.