Hard-to-reach (HTR) energy users encompass individuals who are physically difficult to reach, underserved, or challenging to engage and motivate in demand-side energy programmes. Given a mix of societal challenges (e.g. inequity, energy poverty, decarbonisation, the COVID-19 pandemic), HTR energy users are receiving increasing attention. However, there is a lack of knowledge on the performance of interventions that target (explicitly or implicitly) HTR energy users, particularly from a behaviour change perspective. Our study addresses this knowledge gap, and aims to provide a systematic ex-post comparative cross-country assessment of nineteen case studies, implemented in eight countries. From a methodological point of view, our study explores and tests the usefulness of applying the ‘Building Blocks of Behaviour Change’ (BBBC) in assessing the extent to which interventions employ design and implementation practices that are known to drive behaviour change. Our findings reveal that interventions perform well with respect to the Audience, Behaviour, and Delivery building blocks, but show room for improvement in the Content and Evaluate blocks. Assessing the BBBC framework reveals promising results in terms of credibility, confirmability, transferability, and reliability; however, limitations and uncertainties are also present. Considering the exploratory methodological nature of our study, the results highlight numerous context-specific factors that frame our findings and the suitability of the research approach. We underscore that greater attention must be paid to both the integration of behavioural science methods into HTR interventions, and the systematic analysis of heterogeneity in future HTR-related energy research.