Energy storage is required for the mass implementation of renewable energy sources into our energy mix. Smart latent thermal energy storage (LTES) coupled to heat pumps has been identified by the European commission as a key enabling technology. Due to the high potential of LTES systems, a large amount of research has been performed on storage materials, heat transfer enhancement and implementation of LTES in specific applications. However, this large volume of research has not lead to characterization methods since a fundamental understanding of the operation of transient heat exchangers is lacking. The present proposal develops a characterization method for LTES systems operating with gas/liquid heat transfer fluids (HTFs) based on a description of the storage material and HTF phase transition interactions in terms of dimensionless energy fraction. The result of this approach is a universal theory for testing and characterization of LTES systems. The knowledge developed in this project will allow researchers to compare LTES heat exchanger geometries and to implement LTES systems in software for energy system simulation for techno-economic and life cycle optimization. Furthermore, the models developed describing the fundamentals will serve the basis for future LTES design models. By enabling the implementation of LTES heat exchangers in the energy systems underpinning our society, the results of the project will be a major step forward towards a sustainable future.