The amount of solar power in winter drops by 90%. Solar energy could provide a larger share of global power if there was a way to store all of the worlds energy usage for months. The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. Thermal energy can be used for a broad range of applications such as domestic heating, industrial process heating and in thermal power processes. One promising way to store solar thermal energy is so-called molecular solar thermal (MOST) energy storage systems, where a photoswitchable molecule absorbs sunlight and undergoes a chemical isomerization to a metastable high energy species. Here we present an optimized MOST system (providing a high energy density of up to 0.4 MJ kg−1), which can store solar energy for a month at room temperature and release the thermochemical energy "on demand" in a closed energy storage cycle. In addition to a full photophysical characterization, solar energy capture of the present system is experimentally demonstrated by flowing the MOST system through an outdoor solar collector (≈900 cm2 irradiated area). Moreover, catalyst systems were identified and integrated into an energy extraction device leading to high temperature gradients of up to 63 °C (83 °C measured temperature) with a short temperature ramp time of only a few minutes. The underlying step-by-step mechanism of the catalytic reaction is modelled in detail using quantum chemistry calculations, successfully rationalizing the experimental observations.