Adoption of new technologies necessitates changes in behaviours, attitudes, and social norms that cannot be sufficiently explained by a single predictor or group of predictors, particularly in heterogeneous populations. Aiming to add a greater understanding of intention to adopt Rooftop Photovoltaic (PV), this paper utilised an established population segmentation tool, the Sinus-Milieus and the Theory of Planned Behaviour (TPB) to account for population heterogeneity of the sample of German homeowners (n = 1800). By employing partial least squares structural equation modelling, we confirmed that attitudes, social norms, and financial evaluation significantly influence adoption intention at the population level. A multi-group analysis revealed noteworthy variations in the relative impact of attitudes and social norms on adoption intention across different milieugroups. Notably, even milieugroups sharing the same broad social status and/or values, as defined by the Sinus-Milieu tool, exhibited significant differences in the influence of attitudes and social norms on adoption intention. Moreover, a Kruskal–Wallis H-test indicated significant differences in the reported level of adoption intention among milieugroups, with the Leading milieus showing the highest intention and the Deprived the lowest. Differences were also observed for the determinants of social norms, attitudes, environmental concern, and novelty-seeking behaviour. Our findings confirm the relevance of considering potential adopters as a heterogeneous group in terms of overall lifestyle characteristics. Tailoring communication strategies to target the underlying motivations of specific milieus can lead to more effective interventions.