A few comments about HPWHs: I helped install four different brands as part of a study for the BPA.  We previewed sites to make sure the installation would be viable and to specs, and we gave them to the homeowners in the study so we had no profit incentive for sub-optimal installations.

These were Chinese heat pumps containing climate-toxic refrigerants. 10% were DOA and required replacement or onsite disassembly and factory phone assistance to run. The digital control panels look to be a failure-prone black box. The GE models , apparently being purchased from Chinese manufacturing fairs, were changing so fast that at one point the online manual, boxed manual, jacket sticker, and actual control panel were all different! One brand had a couple of fires, so was withdrawn from the study. (Also pointing up how meaningless UL listing is these days.) Caveat Emptor.

As usual, the majority of licensed plumbing/mechanical contractors/dealers we hired to do the installs never seemed to have read the installation manuals, so I had to correct many significant installation errors, with some tense moments along the way. Just one example; getting them to use 3/4" pipe instead of 1/2" for the condensate runs was a major push (have they never cleaned the slime out of a plugged AC condensate drain?) How installed HPWHs actually function after unsupervised plumbers are done with them would be a separate study! Caveat Emptor.

 Our study participants were a mixed group of early adopters and free water heater households. A good number of them didn't really understand how to make sure the units stayed in high-efficiency operating modes. How can we be sure what mode these units are in, and how long will it take to discover if they error and default to resistance heat?

The as-installed efficiency, reliability, and lifespan of HPWH units is an open question, as is what happens to their refrigerants and electronics when they die.

Where to install a unit flowing ~100 cfm of cold, dry air all year? Envelope pressure imbalances are easily induced by flows of this magnitude, and not many places in a house can just have the air moving around the room with any degree of occupant comfort. The fans are not capable for any useful amount of ducting in most situations. In addition, the noise and vibration were problematic in many locations in the home.

My take on HPWHs is that they probably belong in a seldom-used open basement or semi-conditioned garage, on a concrete floor against a concrete wall where nobody minds the cold air and noise. Install it yourself or stand there with the installation manual and a handgun while the contractors do their wretched best. You could also go to a casino.

Low-climate-forcing hot water production should involve more than simple cost effectiveness calculations. "Cost effective" by what standard and compared to what? PV inverters are the weak link in PV systems (does value engineering consistently undersize them so they die young?) Full life cycle and climate costs may change the numbers, and refrigerants are something to use very carefully.  Are real-world HPWHs really the fully effective solution over the life of the tank, not to mention heat pump? Maybe just install a larger PV system and oversize the inverter a bit so it doesn't work so hard?