I currently have 2x 50 gallon electric water heaters installed side by side in our garage to support 3.5 bathrooms 2 dishwashers and laundry. I’m considering switching to Rheems 120v heat pump water heater, my question is do I need to replace with 2x 50 gallon heat pump water heaters or 1x80 gallon?
I seriously doubt you'll get enough performance out of a unitary HPWH
Given that you have a very high-draw house to the point where you have 2 fairly large electric water heaters in series, I seriously doubt you'll get anywhere near enough performance out of any single unitary HPWH. To drive home the point, the 50 gallon Rheem Professional Prestige is specified at a 63-gallon FHR, something that requires an 80 gallon 120V ProTerra to beat at 72 gallons of first hour recovery. Given that you have the two tanks plumbed likely in parallel, you'd be pushing it to get enough performance out of anything short of 2 80 gallon 120V ProTerras, or 2 50+ gallon 240V ProTerras at 67 gallons of first hour recovery apiece for that matter.
Enter the split-system HPWH, stage right
However, this doesn't mean you have to give up on having hot water available when on generator power, provided you're willing to spend some on a better hot water heater. As it turns out, unitary HPWH designs are severely limited in performance by the fact their heat pump unit has to be small enough to fit atop the water heater. Removing that limitation by moving the heat pump part outside gives you a split system HPWH architecture, which gives you several benefits:
- Split systems don't put a parasite load on the house heating system in winter (this can be avoided with a unitary HPWH by way of a duct kit, but that runs into limitations on intake air temperature)
- Split systems can be made large enough to provide ample capacity even for high demands (the base model Sanden SanCO2 has a 69 gallon First Hour Recovery on a 43 gallon tank, and can push a 115 gallon First Hour Recovery on the 83 gallon tank model, all without drawing more than ~3kW of power, guaranteed)
- Split systems can be built for low-ambient operation much more easily due to the extra capacity possible in the form factor (the SanCO2 units are happy down to -25°F, and deliver a COP of 2.6 at 17°F, temperatures that'd completely freeze out a typical unitary HPWH were it to try to ingest air that cold).
The primary downsides, of course, are cost and avaiability: the SanCO2, while well-proven in its home market of Japan, is relatively new to the US market (and thus not carried by very many distributors), and the 83 gallon version (which'd be what it'd take to give you sufficient capacity) costs about the same as 2 of the 80 gallon Rheem heat pump hybrids. It also requires a bit of additional plumbing and wiring compared to a unitary design; all the refrigeration bits are in the outdoor unit, though, so it's still within the realm of what a qualified plumber can tackle.
Since what you have is more than what you are contemplating I would stick to these.
Look for a model of heat pump that is not integrated with the cylinder. There are many such brands and models.
Unless you buy a grunty one, it may well heat more slowly so you will want more storage.
We have two 300l cylinders, which is 80 gallons each. This is for 4 of us. And we have a 3kw heat pump on each. Part of the reason is that when all kids have left, we can turn one off. It works well for us in tetms of availability of water. We also have solar power so I use home automation to only heat when we are generating solar power.
This is perhaps not the answer you were looking for, but that number of bathrooms and dishwashers doesn't seem like it should require that amount of hot water. Are you sure that one 50-gallon heater would not suffice? If not, why not? If you have high-flow devices, like shower heads, consider replacing them with WaterSense models.
If those are in series you may consider replacing the first unit (the one that gets the coldest feed) with an 80GAL heatpump and keeping the second as-is.
HP are much more energy efficient bringing from cold (5°C) to lukewarm (40°C) than to bring from lukewarm (40°) to hot (60°) so doing as suggested you maximize the efficiency of the HP and in case of extra load the traditional heater will give the extra boost needed.
Also remember HP are slower compared to traditional boilers so allow for some extra capacity.