The performance, monitored over 2 years, of side by side detached Welsh Passive is presented. The houses which differ in terms of glazed area, the use of thermal stores, area of installed PV and occupant behaviour provide a valuable insight into different strategies to achieve low and near zero low carbon performance in social housing. Dwelling 1 and Dwelling 2 achieved average net annual carbon emissions of 9.4 kg/m(2), 24.0 kg/m(2), approximately 13%, 33% of a typical "building stock" UK house. The risk of summer time overheating in the dwellings which different glazed areas is analysed in depth. Dwelling I certified to Code for Sustainable Homes Level 6 did not achieve true onsite zero carbon performance, the installed PV systems in each dwelling would need to be approximately 6 kW peak to meet this target. Occupant electricity consumption behaviour and appliance choices have a great impact on the energy performance of the houses. With current carbon intensities electricity consumption in UK passive houses needs to be limited to approximately 30% that of the current UK average household if primary energy and zero energy targets are to be achieved while limiting PV size to below 5 kW peak. (C) 2014 Elsevier B.V. All rights reserved.