To face climate changes and energy dependency, governments encourage their industries and communities to increase the share of renewable energy (RE). However, the RE production is mostly inflexible. The risk of unmatching electricity market grows. Tools such as power plant flexibility, import/export, demand management, storage and RE curtailment are developed to handle this problem. This study focuses on the energy storage mix required for the energy transition of the electricity system to high RE shares. An hour based model is developed in order to optimise the renewable energy and storage assets by maximising the energy return on investment (EROI) while respecting power fluxes constraints. The model is used to quantify the storage needs for the energy transition of Belgium. An in-depth analysis is performed for four scenarios. Depending on the RE deployment and nuclear share, EROI between 5 and 10.5 are obtained. Large scale storage is required as soon as the energy mix has more than 30% of RE. With more than 75% of RE, power to gas becomes unavoidable. This study highlights that curtailment can be limited to less than 5% of RE production. These values are the result of the optimum between increasing storage, RE capacity and curtailment.