This paper presents a novel energy storage architecture aimed at providing the energy autonomy of battery-free wireless sensors powered by energy harvesting. It is based on a concept of adaptive storage using ultra-capacitors and allows meeting contradictory compromises: fast initial startup (small capacitance value) to be able to supply the load as quickly as possible, storage of a large amount of energy (big capacitance value) to increase the energy autonomy of the load and providing a pre-regulated voltage. Compared to previously published structures, the proposed architecture is much less complex and exhibits 92% energy utilization efficiency.