There is no thermostat for the PV panels. The summer power output of the panels comes into equilibrium with the heat loss of the tank at 80-85C. I did add a 93C thermal fuse to augment the safety of the PTV value.

I have a mixing valve on the output of the water heater to mix with cold down to about 50C. This lets you get around 2x the nameplate capacity of hot water out of the tank so there is enough to bridge a cloudy day.

To get the full capacity of the panels, it necessary to choose a water heater element that matches the source impedance(Vmpp/Immp) of the solar panel array.