Dump the plumbing and use PV to heat water?

[Xringer]

Now that's a nice system!

Since it's only for two people, why not use AC power right out of the enphase inverters?

But, only use 120vac. That way, it will only draw 1.1 kW of power. (13 ohm element 9.23A)
(Use a standard 120vac 15A circuit outlet).

Only apply power to the heater when the sun is bright and your Enphase system is putting out at least 2kW.

That way, your PV system will not send power out to the grid, but use it in real-time, heating your water.

Heck, I tested with 120AC once and it worked pretty well.
I might have used an X10 powehouse Appliance module.
It can handle a 15A resistive load.. ($10 to $15 on Ebay).

The hard part would be to build a fall-back device.
If there is no sun, and the tank is starting to cool off,
kick on the 1.1 kW anyways.. But limit it with a timer. Don't need hotwater at 2AM..
At that power level, won't cost much.. If the sun comes back out..

My tank has the A7 ASHP to use when the sun isn't out.
It's been rainy here, so the A7 ran yesterday and it's going to run again today.
Right now the tank isn't really hot, but it's good enough for a couple of showers.
At 13:00 the timer will send 120vac power to the A7, if the sun hasn't warmed it up,
the A7 will turn on and run for an hour or two. The timer removes AC power at night.
Since we are retired, we don't need real hot water in the early AM.

If my Solar panels disappeared, or were hail damaged, I would use 120AC
on a timer to run the A7. If the A7 died, I would use 120AC directly on the 13 ohm heating elements (via timer).
At 1.1kW, I figure usage would be 4 to 5 kWh per day max. That's still not a lot.