State-of-the-art methodologies for building-level seismic-fragility estimation of buildings typically involve extensive non-linear dynamic analyses of refined Multi-Degree-of-Freedom (MDoF) numerical models. While this approach provides a detailed description of the seismic performance of the structure, simplified and time-saving procedures can be preferred for practical applications. For instance, equivalent Single-Degree-of-Freedom (SDoF) models are often used to reduce computational cost, especially in large-scale seismic risk assessment. Yet, further research is needed to assess the effectiveness of these simplified modelling approaches compared to more refined ones, particularly when energy-based quantities – rather than, or in addition to, more traditional displacement-based peak quantities - are employed. This paper investigates the influence of different refinement levels of modelling approach on energy-based seismic fragility analysis of existing Reinforced Concrete (RC) frames. To achieve this goal, a four-story moment-resisting-frame structure is selected. Seismic response analysis is performed through Non-Linear dynamic (Time History) Analyses, NLTHAs, using alternative numerical models of the structure, including both (i) alternative simplified equivalent SDoF systems and (ii) a more refined MDoF model. For each modelling strategy, the relationship between different energy-based quantities and displacement demands is investigated, and a fragility estimation is conducted through both displacement-based and energy-based approaches. The results presented in this paper can support the decision-making in selecting the most suitable modelling approach for different end-users/stakeholders.