This can be illustrated by considering the diagram . The composite wall thickness ∆t, relative to the radius R of a full-scale solar thermal tank, is shown notionally in the figure. Logically, the edge effect region for heat loss is negligible in comparison with the regions or areas afforded by the sides, top, and bottom portions of the tank. When the wall thickness, ∆t, is very much smaller in comparison to the tank radius R, or:
     ∆t/R <<1