Three 10 kWp grid connected PV systems have been realised on the hat roofs of office buildings in Portugal and the Netherlands. Monitoring results of these systems show that (interarray) shading leads to losses up to 14% at a ground cover ratio (GCR) of 0.61 and an inclination of 30 degrees, when the fraction of diffuse light is relatively large. A large facade behind the system may cause losses in the order of 5%. A simple model has been constructed which explains these losses in terms of loss of aperture. The model shows that at high GCRs it is best to place arrays at an inclination which is as low as possible, in order to obtain the highest possible yield. By means of a case study it is shown how to optimise system size for lowest kW h price, given a specific available area.