Which water heater will most likely be more useful in my future, not yet designed, low/no-CO2 geothermal space conditioning and water heating system - electric tank (actively heating or passively storing/insulating), gas-fired tank (actively heating or passively storing/insulating), electric tankless, or gas-fired tankless?  In the future I'll have a system engineered to generate as little CO2 as possible while still allowing a "normal" lifestyle.  This might be a solar voltaic-powered geothermal unit incorporating a desuperheater and backed up by… (which of the above would best fit here)?  In the now, my gas water heater sprung an irreparable leak.  So, I need to buy a water heater NOW but want it to be a component of the FUTURE system (so I don't have to re-buy then something I could have bought now if I knew what I was doing).

I need to heat about six months of the year, cool about four, and neither about two (yes I know I could reduce CO2 by not cooling, but with pollen and spore allergies it's a good idea to keep windows closed and run air through the air-handler's filters).  Comments on the following thoughts???

Maybe electric tankless would demand too much electricity from the solar voltaic (assume I'm not grid-tied), so electric on-demand probably would not be useful in the future system.  If I bought it now I'd have to toss it then.

Not combusting carbon-based fuels in the future system rules out gas-fired tankless - unless it would hardly ever fire because the DSH kept the water hot enough year around.  But my understanding is that geothermal will not provide efficient hot water in the ten winter months here, such that the gas-fired tankless would have to do most of the work in those ten months.  Is that correct?

An electric tank model could be used as backup when the geo wasn't warming water during the winter or to get the water temp all the way up if the desuperheater didn't heat the water fully in the summer, right?  But by the time I'm able to install geothermal, electric tank water heaters might have been improved, and between now and then, a gas-fired tank water heater would produce less CO2 than an electric tank unit powered by my current coal-derived electricity.

So I think that means the water heater that produces the least CO2 in the short run but could also be useful in the long run best with a geothermal unit is a gas-fired tank that in the future system would store the water from the desuperheater until the future electric tank unit calls for it, adds a few degrees to get it up to temp, then sends it to the shower.

'Tanks (get it?!),

Gordon

My vote is electric tank
mainly because it can be turned into your buffer tank down the line ( or not ) and you can make any other choice , or simply buy another electric tank at that point
while most studies show gas having a much smaller foot print when you consider where most people get there electric from .
long term that should mostly be handled by the pv . and you can usually contact your electric provider and opt to buy no coal produced electricity

In the winter months your desuperheater will produce hot water very effeciently it just wont be "free" ie recaptured ( other wise wasted ) as it does in the summer

Thanks GeoFan.  For GeoFan or anyone...  If geothermal produces enough hot water in the winter, does that open the possibility for a gas tankless to be bought at this point (my goal is to produce hot water with 80% or less of the CO2 output of my now defunct 12 year old gas water heater)?  My thinking is that once the PV-powered geothermal is installed, if it provided enough hot water in the winter, maybe the gas tankless would only have to kick on infrequently.  Maybe infrequently enough to meet my goal?  Thoughts on this please, or a URL to a calculator/modeler???

I agree that the PV-powered  geothermal water heating system an electric tank would likely be the best.  But if I could guesstimate I'd meet the 80% or less CO2 goal with the PV/geotherm/gas tankless I'd opt for that 'cause it would produce less CO2 than an electric tank powered by coal-derived electricity in the years before the rest of the system was installed.

Solar thermal is the least CO2 intensive way to heat water

GeoFan or Engineer... I'm leaning toward gas-fired tankless for double back-up reasons. Electric water heating would augment either a solar or geoexchange hot water system well on a daily basis (first backup). It would either bring warm water fully up to temp if necessary or, in the case of solar thermal, bring less warm water fully up to temp after a few days of clouds. But, my solar thermal (with electric pumps) or PV-powered geoexchange would be grid-tied and so I don't think I'd have hot-water from either if both the sun went away for a few days and there were a power outage. Battery-backed up gas-fired tankless gives me a backup under this second scenario. Does that seem sensible?

Off grid, maybe so.

Using PV-produced electricity for resistance heat (space, water, or clothes dryer) is likely to require a huge PV system and may result in a payback time of decades if not centuries. Wind or Microhydro is another matter but they are site-sensitive and maintenance intensive.

Building any off-grid system hefty enough to handle a heat pump, geo or not, will be pricey. Most I've read of use wood or gas for heat and backup hot water, cooking and drying clothes and are in climates so as not to need much AC.