AbstractEuropean Union (EU) legislation is increasingly embracing the energy efficiency first (EE1st) principle. This principle seeks to prioritise energy efficiency measures whenever these involve lower costs to society than generators, networks and other energy supply options while achieving the same outcomes. This study contributes to the quantitative evidence on the relevance of EE1st by modelling the impact of moderate to ambitious end-use energy efficiency measures on energy supply and the associated system cost under a net-zero greenhouse gas emissions constraint by 2050. These measures focus on the EU building sector and include both building retrofits (e.g. wall insulation) and efficient products (e.g. lighting). The results indicate that implementing more ambitious energy saving measures reduces the total electricity, heat and hydrogen capacities needed to achieve the net-zero target. Reducing energy use in buildings by at least 21% between 2020 and 2050 is essential to avoid excessive energy supply costs. This requires actions that go well beyond business-as-usual trends. Reductions of around 30% could be justified on the grounds of (i) high fossil fuel prices and (ii) multiple impacts (e.g. health benefits). Overall, the outcomes provide reasonable justification for the EE1st principle. To put the principle into practice, policy actions such as doubling building renovation rates and setting higher energy efficiency targets are key.