The solar storage tanks display their storage capacity not until the collector temperatures are higher than the desired consumption temperature. The higher the „over-temperature“ in the tank is the higher the storage capacity.

Assumed the heating temperature is adjusted at 60 °C and the back flow has 50 °C. A high temperature collector with low volume flow feeds a buffer tank (start temperature 60 °C) to 90 °C in one step. For every litre at 90 °C 4 litres of water at 60 °C can be fed into the heating network. A low temperature collector needs 3 times the tank volume to yield the same heat energy at 70 °C. If the heat from the buffer tank is used through a heat exchanger, the usable water quantity will be

much smaller and the cooler buffer of less value because the cooler the storage tank is the more usable heat (exergy) is lost by heat exchange.