Looking for your thoughts on the use of an AirGenerate water heater that preheats the water before it hits a Marathon water heater that boosts the water temperature to 120-130 degrees Fahrenheit. The idea is that the AirGenerate water heater works exclusively in economy mode (heat pump only) maximizing efficiency while the Marathon acts as a fail/safe booster tank that ensures adequate hot water supply in any situation. The two tank system is borrowed from solar thermal water heater installations.

I talked to some home performance contractors recently about efficient water heater solutions in all electric homes. All electric homes are electricity to heat the home and the hot water. Some home performance contractors have run into the issue that their chosen heat pump water heater could not supply sufficient quantities of hot water in homes with 5 or more adults. The recovery of the heat pump water heater was simply to slow to supply hot water for back to back showers. At least of the heat pump water heaters needed to be pulled due to customer dissatisfaction.

One of the contractors came up with the two tank solution: AirGenerate water heater (pre-heater) + Marathon water heater (booster tank)

Seems to me like a decent solution for larger households with electric water heating needs.