This signifies low running costs but can require more boosting. The reason being no electricity is required for moving water from collector to storage tank. It occurs through thermosiphoning a natural phenomenon, heat rises. Water in the panels is heated as the hot water rises into the tank it is replaced with cold water from the tank.
The trade-off here is a bulkier unit having high system weight on the roof. For example, storage tank of 300L can hold 300 kg of water plus collector and tank weight.
Split systems contain roof collectors with on ground storage tanks. Temperature monitoring in such cases occurs with controller and solar pump. If your systems is sized correctly and you understand how to use your hot water boosting can be minimal.
This also helps the movement of water from the collector to the storage tank. Energy use although minimal is present and it is approximately 28 to 60 watts/hour for 8+ hours daily.