To truly reduce environmental impacts in sustainable consumption, lifestyle assessments should be performed for all areas of protection and because human behaviours drive lifestyles, they must be accurately accounted for. Life cycle analysis (LCA) computes environmental impacts across several indicators and throughout the life cycle stages of a product or a service. However, its capacity to grasp human behaviours and their changes is limited. This is especially true when one wishes to assess the potential benefits of behavioural policies based on people’s irrationality such as nudges. This article describes a methodology for the environmental assessment of systems and policies that aim to change human behaviours. To that end, agent-based modeling (ABM) and LCA are combined. While ABM simulates human behaviours and their changes, LCA assess environmental impacts. The methodology is applied to a case study of standard and smart homes use phases. Results show that attitudinal factors such as conformity to others cause significant effects—up to approximately 30% of environmental benefits in the experiments—and should therefore be accounted for. When performing peak shaving (a shift in time of part of the electricity load), the use of a photovoltaic (PV) battery system increases the reductions in climate change impact by up to roughly 25%. However, peak shaving may also lead to environmental trade-offs depending on the metric used. These results show the relevance of combining LCA and ABM when evaluating systems or policies that change people’s behaviours (e.g., nudges). The proposed methodology could enable the assessment of complex systems in sustainable consumption. Evaluating other such systems (e.g., mobility or diet) also constitutes a possible application of the developed approach.