Just an update to this thread. So far, so good. I've done a few things, like insulate the hot side piping, and I designed and 3D printed a duct to go on top of the unit that I'm going to punch through to the attic directly above the heater (The 'Official' Rheem duct is $150, no thanks). The air up there is a lot warmer year round than it is in the garage, so that should give a nice little boost to the efficiency of the unit, not that it struggles in that department. Most days we are using less than 3kWh's of power, some days less than 2. For reference, a while back I was able to determine that I used around 4kWh just to take one shower, I still take at least one a day, as does the rest of the family (2).
I got my $500 check from the EMC yesterday, so the unit has officially cost me $568, not counting the install items, which most of I'd have had to buy to replace it with another resistive unit anyways, so I'm not counting those, but that was about another $150ish. (Pex connectors and tools, expansion tank, condensate pump, etc).
All-in-all, extremely happy with the results.
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