Jake thanks for the reply. Yes I want the tanks to store the hot water from the desuperheater, but this only works in the hot summer months. You don't get much hot water in the winter months which is 70% of the time. The reason I don't want to use electric because gas is much cheaper to run. I was thinking it may be cheaper in the long run to just buy a powervent gas WH than to do electric. I thought about the combination, as you suggested, with electric to store and tankless to provide the extra heat. My concerns is it is already plumbed for a recirc system and I wasn't sure how that would work with a tankless.

There is a LOT of disinformation on these forums about on-demand tankless units, ESPECIALLY the electric units. First, energy cost savings is NOT a reason in itself to make the switch. No reason alone is sufficient. It is the total package, in light of your particular lifestyle, that will dictate the pro-con outcome of the decision making process. First, tankless units save space -- a lot of it, especially electric units that do not need to be vented. In Texas we have no basements, so that can be an important issue. Second, an electric unit is unlikely to blow up, unlike anything using natural gas. Third, there is an endless supply of hot water, which in a successive-morning-shower household like mine, is a really important issue. And finally, yes there is some documented comp fuel savings (electric tankless over electric tanks, or gas over gas), that is really offset by the added initial cost of the unit plus upgraded electrical service or gas supply line sizing. Moreover, there ARE full-house electric units out there that provide plenty of water, unless you plan on taking two showers simultaneously while also doing your laundry and running the dishwasher! And if that's your bag, you can run 2 or more electicals together. Or get a really big gas unit. Bottom line, nearly every nay-sayer of tankless units ignore the pros and focus on a single con - cost, cost, cost. Well, we all don't drive a Ford Focus for a reason. Some of us enjoy our Buicks, Porsches or Camrys even though another car would get us from point A to point B cheaper while using less fuel etc etc etc. Full disclosure: I have a Steibel Eltron Tempra 36 whole-house electric unit. Love it. My electric bill went up nary at all.

The 2-main facts you stated are that you are doing a geothermal liquid-to-air system with a desuperheater to preheat the domestic hot water (DHW). The second fact you stated is that a regular active, as in electrically powered, 50gal domestic hot water tank would meet all your hot water needs. Based on these 2-facts, all that required is a passive buffer tank that is properly plumbed between the geothermal heat pump’s desuperheater and another active 50gal electric domestic hot water tank. That’s it! The reason for the buffer tank is that the desuperheater wants to see a greater temperature differential between itself and the entering water, as will be provided by the buffer tank, without the buffer tank you will have hot water circulating from the active tank through the desuperheater with very little temperature differential. That would be very inefficient. During the summer when you’re air-conditioning anyway your domestic hot water will be essentially free as the heat removed from your home will be transferred to the buffer tank rather than just exhausted to the outdoors as with an air source heat pump. The electric elements in the active tank will either hardly come on or not come on at all, depending on the amount of air conditioning required and domestic hot water consumed. Have a look at the link below for informational reference regarding high quality buffer tanks. Remember that the buffer tank should have no electric elements or electrical connections at all. Your geothermal contractor that has experience doing this should plumb it. A plumber that lacks this experience might get it wrong! Verify the credentials and references of your geothermal installer. Read some of the other numerous geothermal posts I’ve written on this site. I hope this helps. SR Marathon Buffer Tank, see model # MTS50200 (no electric elements)

If you're really sure you want tankless (and you kinda seem to be), I'm not going to argue with you. I'll just say a couple of things and then I'll bow out. Disclaimer: I'm not an expert on these matters. They just interest me and I hope to be more energy independent in the future, so I read up on them. I always welcome corrections from knowledgeable folk. First point: IMO electric resistive heating of ANY kind basically just doesn't play nice with PV, off grid or on. The only time it really makes sense is when you're off grid and your system produces more energy during the day than you can store in your battery. This can also apply to some extent if you're intertied and your utility pays you wholesale or less when you sell your power, essentially penalizing you for conserving. Either way, dumping your excess PV energy into water heaters is one way to store it for later use. However, it should be obvious that to store that energy, you have to use traditional storage water heaters, not tankless. Second point: Peak demand matters. It matters to your electric utility, and it matters to your bill. You probably know more about CA rates than I do, but it appears to me that right now California is more into time of use (TOU) metering than peak demand metering. However, more and more utilities are adopting peak demand metering for residential customers (they've had it for business for ages). So even if you're not dealing with it now, it may well be in your future. Demand based metering watches your power (not just energy) usage. The higher it is, the more you pay for all your electricity. In one demand scheme, you pay a surcharge based on your peak power usage (in kilowatts), In another, the price they charge you for every kilowatt hour you use depends on your peak usage -- the higher your peak, even if it's only once in the month, the more you pay per kWh that month. Let me give you an example of the second scheme. Let's say your house's big energy hog right now is an electric range.. Most of the time, you just use one or two surface units at a time. So your peak demand is relatively low, and you pay (say) 14 cents per kWh most months. But when you cook Thanksgiving dinner, you have all the surface units going, plus the oven, and a microwave. On that one day, you have a much higher peak demand. Your utility takes note. And that month, you pay 20 cents per kWh for EVERY kWh you use all month. So even though that month's usage is only a little higher than usual, the final bill for the month is MUCH higher. Now imagine what could happen to your bill if you add an appliance that increases your peak demand every day of every month. Guess what: when it comes to peak demand, tankless electric water heaters are MONSTERS. Think about it. With its mild climate, California is a great place for heat pump storage water heaters. That's what I'd get if I were you. Not tankless, and definitely not electric tankless. And that's all I have to say about that.