Today, the production of solar and wind energy have become more profitable than nonrenewable alternatives. However, daily and seasonal variations in the renewable energy production as well as large variations in the power demands are two serious challenges for a sustainable energy eco-system. Storing excess power and using this energy in peak times is therefore absolutely crucial, but it requires affordable large-scale energy storage systems. One technology that attempts to solve this problem is closed-cycle MOlecular Solar Thermal (MOST) energy storage. Such systems relies on photochromic molecules or molecular photoswitches where stable reactants can interconvert to form metastable products using solar irradiation as the driving force. Thus, the solar energy can be stored as chemical energy until a subsequent exothermic reaction releases the captured energy. Depending on the storage lifetime, this energy release can either occur spontaneously or be controlled by e.g. a thermal activation, a heterogeneous catalyst, an electric potential, or light. To paraphrase, MOST systems are able to harvest and store solar energy, which can later be released as clean thermal energy for space heating or heating of domestic water. In fact, the thermal energy can be stored without the need for thermal insulation, which enables prolonged storage times compared to alternative thermal energy storage systems.