[nibs]

Xringer, you could just slap a snap switch on the tank that goes open when the tank reaches a high temp, a second snap switch could redirect the power to batts or ???
Cheap & simple, tho with that much pv (grrr) you might need relays.

I used to control my hot tub pump,that way, when we heated it with propane.
Now we heat the tub with wood so we no longer pump, it thermo syphons, now we control the temp by using the stove door.