This review analyzes the inherent scientific challenges of realizing the potential of storing solar energy by photochemical generation of high-energy metastable compounds whose subsequent thermal isomerization releases large amounts of low-temperature (<500 K) heat. Such compounds may be stored at room temperature for days or months, regenerated using sunlight, and may be cycled many times without significant degradation. After highlighting some of the general challenges of solar energy conversion and storage, we discuss how recent advances in understanding the effect of molecular strain on the thermal and photochemical reactivity of small molecules offers new opportunities for a systematic approach to the molecular design of solar thermal fuels, defining the molecular properties which determine the fundamental limits of such a material's performance characteristics.