Energy storage is acknowledged a key technology to meet the challenges posed by the energy transition. Shortterm grid-connected storage, based on Li-Ion batteries, is becoming commonplace but seasonal energy storage at grid-scale will be needed for deep decarbonisation of the electrical power system. Pumped hydropower is considered to be the only mature technology for such applications, but this paper demonstrates that two-tanks molten salts systems, that are used today in commercial concentrating solar power (CSP) plants, can also be considered a mature technology that can be used at large scale for seasonal energy storage. This was established by evaluating the annual heat losses of molten salts tanks using validated models of these systems. The results show that the heat losses in a very well insulated molten salts tanks are around 1 K/day, which would result in good economic performance of the power block even if storage was required for up to months.This work was partially funded by the Ministerio de Ciencia e Innovación de España TED2021-132216A-I00 funded by MCIN/AEI/10.13039/501100011033 and the European Union by NextGenerationEU/PRTR. This work was partially funded by the Ministerio de Ciencia e Innovación de España (PID2021-123511OB-C31 - MCIU/AEI/FEDER, UE; RED2022-134219-T). The authors at University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group GREiA (2021 SGR 01615). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme.