The world has experienced several energy transitions. During the end of the XIX century, coal became the first energy resource before biomass. After WWII, the thermodynamic quality of hydrocarbons made them the first primary energy resource. Finally, today due to global warming threats, humanity starts a decarbonising energy transition from fossil fuels to renewable energy (RE) sources. This study focuses on the energy storage mix required for the energy transition of the electricity system to high RE shares illustrated by the case of Belgium. An hour-based model is used in order to yearly optimise the country RE energy return on investment (EROI). The storage dynamics is studied through its monthly needs. For an 80% RE Belgium, curtailment reaches 3.4%. It induces as much losses as the batteries. At high RE shares, Power to Gas (PtG) becomes unavoidable even if the amount of energy involved is less than 2% of the overall yearly consumption.