I have two 80 gallon indirect water heaters connected to a closed loop solar system. They have auxillary 3.5kW electric heaters in each with standard water heater thermostats and high limit switches.
The plan is to buy two 20A contactors, and use the 120V switched power from a smart-plug to pull in the contactors. I'll schedule smart-plug to turn on in the evening near the end of the solar harvest to make sure the storage temp reaches the electric heater thermostat set-point of 145F. Once they reach set-point I think I can program the smart-plug to shut-down and wait for the end of the next day to kick in.
The electric thermostats are at the middle of the tank, while the indirect coils are near the bottom. So its very likely that the smart-plug will energize at the end of the day and there won't be any demand for electric heat from the thermostats.
I was thinking about hacking a contact sensor to use an aquastat contact that watches the top of the tank temperature, but darn if aquastats are so expensive.