Abstract: A storage tank especially suitable for use in a solar heat system utilizing water to store its collected heat is disclosed herein and includes a main body which defines an internal compartment for containing the water. This main body includes a composite inner layer for preventing the passage of water therethrough, an intermediate layer of thermal insulation having an R value between about R-20 and R-35 and outer layer for substantially preventing the passage of water therethrough. The storage tank when utilized as part of a solar heat system also includes an arrangement for passing water directly into and out of the internal compartment and an arrangement adapted to support at least one heat exchanger within the compartment.