Here's a posting showing what happens when a device (circuit breaker in this case) rated for AC was used to interrupt DC:
Getting ready to mount PV panels and pull wire. I need a breaker for this but not sure what to use. Panels will be in series so the breaker will need to be rated for at least 75 volts DC. I like the breakers I have for everything else but they are only rated for 48 volts DC. I could use an...
diysolarforum.com
You've seen the still photo.
Now watch the movie from Myth Busters.
https://actii.pl/en/ladowarki-prostowniki/979-mpm-pwm-heater-charging-controller-for-solar-solar-panels-actii-ac7391-6931240073919.html Interesting; I was not sure which country you live in by the currency that web link shows / got converstion of cost to USADollars of: $178.13 ... Hope it...
diysolarforum.com
They say this has happened a couple of times with real houses.
All I'm saying, is don't use the thermostat and overtemp device in a water heater for high voltage DC.
Run either AC or low voltage DC through them to control DC switches. Two separate devices, not combining to one relay, so as to not reduce the amount of redundant safety.
Or, easiest these days, just run the water heater off your inverter when you have surplus PV.