Abstract Dunkelflautes, events of low renewable production and potentially high demand, are the most extreme examples of the susceptibility of a renewable energy system with respect to weather variability. Yet, these events currently lack a precise definition in the literature. Based on 41 years of weather data, we explore the resilience of the Belgian energy system with respect to the duration and extremeness of Dunkelflautes or energy drought. Although the hourly average load cannot be covered 89% of the time in our analysis, we find that Dunkelflaute occurrences are generally independent of the energy mix between photovoltaic (PV) panels and wind energy (WE) production (for fractions between 10% and 60% of PV). However, to cope with Dunkelflautes, a system with full PV or full WE provision is very ineffective. While the optimal mix is found to be independent of Dunkelflaute extremeness, it strongly depends on the Dunkelflaute duration: For subdaily or longer than two-daily events, a high WE fraction is optimal, while a high PV fraction is needed for durations of the order of days. More than 56% of Dunkelflautes over Belgium coincide with those in the surrounding countries. Finally, as opposed to weak changes in the climatological average, future climate projections reveal an increase in Dunkelflaute severity, mainly due to a reduction in WE production.