The thermal storage is a key component of any successful solar thermal system. A good thermal storage should allow minimum thermal energy losses while permitting the highest possible extraction efficiency of the stored thermal energy. Despite the many available examples of successful designs of solar thermal storage tanks, the static behavior of the solar thermal storage is not fully understood. Among the many factors influencing such behavior are heat losses, tank geometry, dead zones, and tank wall material. In this study laboratory-scale models with different geometries were built for the purpose of examining thermal behavior during the period after charging. During this period, called the thermal diffusion period, the extraction efficiency was temporarily decreased until natural stratification was achieved. After the thermal diffusion period, the extraction efficiency decreased primarily as a result of thermal losses to the environment and thermal degradation caused by the storage tank walls. Increasing the ratio of length to diameter up to 3 or 4 significantly increases the extraction efficiency. Storages with length-to-diameter ratios larger than 4 are not desirable because the added cost does not result in improvement of the thermal extraction efficiency.