I have solar panels charging deep cycle batteries, on bright days the pannes produce more power than the batteries can take so the voltage regulator on the panes just cuts them off.
This is obviously a waste of power, but also its better to keep the battries at a constant 28V. So the idea with this circuit is to divert the small amout of excess power from the solar panels to the hot water tank. This should result in better charging of the batteries, and free hot water.
Tom