<p>Energy affordability and reliability are crucial for human development (HD). However, energy transitions without fairness can worsen social inequality. In this context, this study develops a comprehensive energy affordability gap (EAF) index to showcase how selected countries have progressed in improving energy affordability. This study includes a sample of 82 high-income (HICs) and low-middle-income countries (LMICs) from 2002 to 2019. An operational “Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS)” was employed to check the achievement of EAF in the selected countries. The generalized method of moments (GMM) regression model was used. The TOPSIS results showed that 10% of HICs maintained their EAF from 2015 to 2019; 58% of HICs showed a revamping in their EAF scores. However, 31% of HICs demonstrated mixed tendencies. Although the EAF scores of LMICs showed improvement in most economies, their improvement rate was stagnant. This study adds to the body of knowledge and helps guide decisions with solid data because it uses a rigorous scientific approach. The GMM method was used to identify the nonlinear relationship between EAF and HD. Accordingly, the empirical impact of EAF had a positive impact on HD, but once EAF reached a certain threshold, the effect became negative. The impact of EAF on HD varies between HICs and LMICs. According to the linearized marginal effect, 27 LMICs out of 34 nations have achieved the threshold value, whereas HICs are behind the inflection point and still optimizing their EAF impact on HD. The study concluded that EAF was negatively correlated with HD in the form of an inverted U-shaped relationship. This study provides policymakers with concrete policy insights by examining the relationship between HD and EAF. In this regard, the results of this study identified important policy insights that can help policymakers reduce EAF while advancing HD.</p> Graphical abstract <p></p>