Heat pump water heaters have a hard time getting water heated those last few degrees, that is why they are limited to 130 degrees and sometimes use resistance elements to get that last little bit of heat in there so I would say to turn down the temp of the heat pump water heater and use it as the pre-heater, or just run the heat pump water heater off solar, it would be a larger investment but it would be cheaper then buying enough PV to run a resistive element.
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