I have two homes in Connecticut, both heated with oil boilers with tankless coils and have just installed my second heat pump water heater. I could have gone with indirect storage but chose this route. Neither has good solar access and propane is equally high and there is no natural gas. If I had solar access, that would have been my first choice. The first unit I installed three years ago before GE and others were offering combined (heat pump / tank) units was a unit made in Maine by Nyle called a Nyle 1 and connected it to an 80 gallon storage tank due to five people in the house. I put a switch to block electric operation of the backup electric and have never turned it on. The larger the tank, the less the need for back up because these only recover at about 6000 BTU's per hour, so if you want to go solely heat pump, the tank must be large enough for your family. Electricity here is 20 cents per KWH but the overall efficiency of running a boiler on maintained temperature is so low, this really pays off unless you have a very high efficiency condensing boiler with indirect storage. In my case, I piped the input to the water heater to the output of the tankless coil and re-wired the aquastat on the boiler to no longer maintain temperature, only to fire on heating demand. So, in the winter, there is some pre-heat.
I have looked at the GE unit and others before buying the one for my second house and decided again on the add-on heat pump for several reasons. The GE unit is attractive because it is stainless which will last a long time but the 50 gallon capacity is too small and I believe, the electric would be coming on too often unless you were really frugal with use. There are some other heat pump units made in 80 gallon capacity but they approach 2K in cost but not sure if they are stainless. But, I wasn't impressed with the transfer method of these units. Most just wrap the tank with the refrigeration coil (condensor) instead of direct immersion. What I like about the add - on units is that it uses a tube in tube counterflow condensor with a small circulator which is much more efficient even with the small extra electricity used by the circulator. I also felt that if the tank failed, I would be out my entire 2K and couldn't transfer the heat pump to a new tank. But, also I liked the idea of the free air conditioning in the summer being ducted upstairs to my kitchen. For the second unit, a Nyle Geyser-R, I bought the duct plate and run the output through flex duct to a register in the kitchen and make up air returns through another vent. In the winter, I close the vent, disconnect the flex and pickup the heat from the room as the boiler is right next to it. For this installation, I also changed the boiler to non-maintaining temperature and allow it to pre-heat in the winter. Again, I have not allowed the electric to come on in the 80 gallon tank I bought from Home Depot.
So, depending upon how large your family is, the size of your tank, how you diversify your use will change the amount you save by not using electricity directly for backup. The GE units that you are considering allow you to select the mode if you want to try without using the backup portion. All of these units whether add on or integral require a condensate drain and if you can't drain by gravity, you have to add a condensate pump and tubing to a drain. Make sure you put it in a room large enough to get the heat exchange. These cannot go in a closet.
A little about efficiency, all of them that use the better refrigerants such as R410, 411, etc or Pureon have a high COP but the COP changes drastically in the heat cycle and depending upon the ambient temperature. For example, if you had this unit in a warm garage as people do in Florida, the COP is close to 4.0 all the time. In my New England basement, the unit will turn on when the bottom of the tank is cold from make up water and with the room temperature at about 65, the COP is about 3.5, but as the water gets closer to 120 degrees, the COP drops to 2.8 for the unit but you have to add the tank loss and in my case the circulator loss so overall seasonal, all of the units end up at around 2.5 for a normal basement. But, if your basement is too cool, say below 50, your COP will drop again. So, instead of getting 3412 BTUs per KWH, I am sure to get 8500 BTU's for my 20 cents per KWH. My boiler which is quite old, no damper, probably has an AFUE of 60% for DHW only but there are all those jacket and flue losses when the unit is off but maintaining so I figured overall about 50% of my gallon of oil (138000) BTU's is lost. So, I might pay 3.75 for a gallon of oil 69,000 BTU's net. 69,000 / 8500 = is about 8.1 KWH for the equivalent energy which at 20 cents comes out to $1.62, quite a savings. So, with an old boiler, you will save money, but for your 2K investment, it might take two or three years to pay it back. I figure with the current oil prices, I am saving about $110.00 per month in oil for a family of five but my electric bill went up in the winter, spring and fall by $45.00. I don't count the summer because of the window air conditioning unit in the kitchen is probably running less.
But, I get some free air conditioning and dehumidification for the months of June, July, August and September which is nice here in Connecticut. What else have I noticed? The basement is cooler and quieter in the summer. But, it is also cooler in the winter as the boiler is no longer maintaining temperature. I look at it as the AFUE of my boiler has improved in the winter since my heat pump is picking up jacket losses but in mild weather, the boiler actually cools off so flue and standby losses go way down. I had one accident though in one of the houses. I forgot to tell the oil company that I installed the heat pump water heater and they came to fill up the tank in the summer assuming summer domestic hot water use. The problem was the tank was already full, so they over pressurized the tank, blew the tank gauge plastic off and spilled a small amount of oil on the floor and a small amount sprayed on the ceiling and wall. They were very nice about it though and repaired the gauge, cleaned up the oil and I apologized profusely for not letting them know so they didn't charge me for the oil or the clean up. But, it is nice not to get an oil bill in the summer and I kind of look at it like I am doing my thing for the environment while saving money at the same time.
If you have any other questions, I would be glad to help and I also got considerable advice from Nyle in Maine from their head engineer, Don Lewis (nyle.com)