The hybrid units likely have an HX with refrigerant too. I wonder how they would deal with 100C?
I've been thinking about this problem for a while. It seems like any system without
a double-wall/(middle wall vented) using refrigerant to heat water, could fail
and inject high pressure gas into your storage tank and/or your house water lines..
I hope my 100 PSI T&P valve will do the job for R22..
If I used the elements in my tank, I would not run them at 3.5kw.
But, even at 1,000 watts. Overheating might be a problem.
So, if I decide to hookup power (even my 800w of PV), I'll do some testing
to see if it's possible to over-heat my 40 gallons of water (using low power)..
If it is, I'll follow the AirTap no extra power rule..
With the limited amount of solar (5 hours a day max), PV might be a safe bet.