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• Synthetic and natural ester dielectric oils are employed as nanofluid carrier liquids. • Fullerene C 60 and magnetite nanoparticles are dispersed in both oils. • AC dielectric breakdown of the oils and nanofluids is statistically analysed. • Fullerene tends to reduce AC breakdown voltage in the natural ester but not in the synthetic ester. • Magnetic nanoparticles in both esters enhance the AC breakdown voltage but in a different measure. Ester-based electrical insulating liquids constitute an attractive alternative to widely used mineral oils, especially due to their ecological benefits. In this study, we present an experimental comparison of AC breakdown voltage (BDV) in both natural and synthetic ester-based nanofluids with various concentrations of fullerene (C 60) and magnetite (Fe 3 O 4) nanoparticles. The BDV measurement was conducted with an AC voltage rise rate of 2 kV/s according IEC 60156 standard. While the suspensions of magnetite nanoparticles exhibited increased AC BDV for all samples tested, the fullerene-based nanofluids exhibited the enhanced AC BDV only for two among six nanoparticle concentrations. Accordingly, we present a comparative view of the electrical breakdown development using statistical descriptors. This study uses Weibull and normal distributions for statistical analysis to estimate AC breakdown voltage at different cumulative probability levels 1 %, 10 %, and 50 %. The parameters of theoretical distributions are fitted to the experimental data. Subsequently, statistical test of conformity to the distributions, and their skewness and kurtosis are analysed. The analysis reveals that AC BDV of nanofluids increases with the increasing cumulative probability more than those of pure esters. It is shown that the addition of fullerene nanoparticles tends to reduce AC BDV in the natural ester but not in the synthetic ester. The presence of magnetic nanoparticles in both esters yielded an enhancement of the AC BDV but in a different measure. [ABSTRACT FROM AUTHOR]