3100 SF single story slab on grade, ICF walls, 10" SIP roof, 20,000 gallon rain water, manifold insulated PEX plumbing, low flow fixtures. Not yet dried in, ALL ELECTRIC in Austin, Texas. (But with Pedernales Electric, so no rebates.)

I've taken so long deciding things in such detail, and yet I still don't have a clear direction on water heating! So many choices and no clear best. On one hand I hear that water heating is the best bang for the solar buck; while on the other hand I hear, even from a vendor offering both PV and high end water heating, to avoid the maintenance issues of solar hot water and put that $7,000 into more PV.

Retirement house is for two people, but designed for future uses by family of four or more. My current leaning is to a super-insulated electric tank, such as a 40 gallon 3800 watt Marathon marathonheaters.com.

Can anyone help me with a basis for feeling content going one way versus another?

In a cooling dominated climate like Austin (or anywhere electricity is your primary utility source) you'd do well to look at compressor-based heat-pump electric tanks that draw the heat into the tank from the surrounding room air with COPs greater than 2  for an effective EF of 2-2.5.  (The state of the art may be better than that by now).  Even a superinsulated tank will have an EF less than 1.0, always!  And the peak load to the grid is well under 3800 Watts for any of 'em.

As a side benefit (but still measurable), it reduces the cooling load since it uses less than half the total power, and the room where the standby losses are released to are where the heat is redrawn from , dramatically reducing the net delivered to the room, compared to those of a superinsulated tank all end up in the room, and the energy transfer is all one-way.

See:

http://www.energy.wsu.edu/documents/building/res/ht_pmp_water_htrs.pdf

The difference in operating cost between the Marathon and a heat-pump HW heater is large- a factor of 2 or more.  Then when you compare the price of one of those to the cost of a solar HW system sized to deliver 50-70% of the load (for an equivalent reduction in power use) it's a bit of a no-brainer.  Bang-for-buck it's pretty good- a bargain compared to straight ahead solar thermal.

If you're planning to spend the difference of forgoing solar thermal for PV, from a solar-efficiency POV, you're starting with maybe 15% efficiency with silicon cells, but using it a COP of say 2.5  with a heat-pump HW system you get an apparent efficiency of (15% x 2.5=) ~38% compared to ~50% for flat panel thermal- it'll take more (and more expensive) panel area to make up the power use, but still less than half the PV required for a superinsulated electric tank.  If there is a minimalist low-cost solar thermal system with the storage/panel ratio designed with lower panel temps in mind that could be used as pre-heat ahead of the heat-pump HW heater you could get better than 65%+ efficiency out of the smaller, lower temp solar thermal, spend the difference on PV, you can probably do OK efficiency-wise too. 

It takes a sharp pencil & all the particulars to determine the most solar-effective & most COST-effective solutions, and where the crossovers are.  But whichever way you go, the heat-pump hot water will likely be the single most cost effective component.  (In the ideal world somebody would make a solar tank with a heat-pump backup rather than electric resistance.)

Stretching even further...

There are vendors of PV/thermal hybrid panels now which run at lower temp offering significant cooling benefits to the PV which enhances the PV's operating efficiency. eg:

http://www.solarroofs.com/  

PV efficiency is usually rated at 25C, but summertime in Austin you won't see PV collector temps that low when the sun is high in the sky, and the performance falls off fairly linearly with temperature.  At 65C (a typical mid-day roof temperature on a hot July day) a silicon PV cell rated at 15% at 25C is now barely scraping 11%, but if kept below 45C by a HW pre-heating system it'll hit 13%  See:

http://alumni.media.mit.edu/~nate/AES/PV_Theory_II.pdf

So in some ways, a hybrid PV/thermal panel system is preferable in a cooling dominated climate to using seperate thermal/PV, even if the thermal performance is limited to lower temp operation by being un-glazed. As pre-heat to another heater it's efficiency would still ROCK in the summer & shoulder seasons (may meet all of your HW needs and then some) even if it delivered little in winter.

OK, have I confused you enough for one post? :-)

Thanks so much, Dana1, for those wonderfully informative and interesting, and not altogether confusing, references and ideas.

I was at one time planning on a heat pump water heater (HPWH) or a desuperheater on a geothermal heat pump. Budget and geology nixed the geothermal. I didn't find good information on HPWH and lots sight of it, but now it looks viable again, thanks to you. The HPWH handily wins a 13-year life cycle cost analysis over everything else. I also like the simplified installation and no roof penetrations.

I would also add this detailed and lengthy analysis of heat pump water heater effectiveness to the mix.
http://www1.eere.energy.gov/femp/pdfs/FTA_res_heat_pump.pdf

The Solar Roofs system doesn't seem to be a hybrid system in the sense you used the term (where water or other heat exchange media would serve to cool the PV cells). They are primarily water heating, with an add-on small PV panel to run a water pump. If you know of a hybrid in the other sense I'd be very interested.

Thanks again for your time and thoughts.

There are a few out there (those were from a quick search- sorry! :-) )

Sundrum has a panel hybridizing  system more like what I was talking about:

http://sundrumsolar.com/index.php/home

http://sundrumsolar.com/index.php/products/34-all/51-sdm100

http://sundrumsolar.com/index.php/products/34-all/49-performance

(In Austin you'll beat the Massachusetts 55-60% net solar efficiency cited.)

http://sundrumsolar.com/files/documents/SunDrum_Total_Energy_Data_Sheet_Rev_C.pdf

I don't know why I didn't think of them first, I drive within 200 yards of their HQ on my daily commute! :-)  There are others out there as well- I'll keep searching...

Another usually cost effective hot water heating technology is drainwater heat recovery heat exchangers installed downstream of the shower drain.  If configured to pre-heat the cold water stream to both the cold water and the water heater energy used for showers (but not baths) can be cut in half or more. (With very low-flow shower heads the recovery efficiency can be over 70% with some of 'em.)  Tub filling & other batch draws get zero benefit, since the drain isn't flowing as the water is drawn, but for showers they're great!

In some instances they can pay for themselves up-front in reduced water heating equipment sizing:  With showers being typically 40%+ of a family's hot water use, it cuts the size of the necessary solar thermal system needed by 20%+. (eg: You can manage 60 square feet of Sundrum instead of 75, etc.)  And by returning 50%+ of the heat during simultaneous flows, it roughly doubles the first-hour capacity of the tank HW heater allowing you to downsize on capacity (you get 4 showers in a row out of a 40 gallon tank rather than 2, no need to buy a super-tanker to keep up.) 

This is tough to implement in slab-on-grade though, since it takes at least a 3-footer to get significant benefit, and they need to be mounted vertically to work.  There are 2-footers for 4" drains that can stuff into most crawlspaces though, returning more than 30% of the heat at 2.5gpm flows, or 30" long versions for 3" drains with similar performance.  But 24-30" is way more than you get in slab-on-grade unless you design in a pit to accomodate it (which also adds to the up-front expense.)

I am about 2 or 3 years behind and about 40 miles southwest of you with a similar size (if I have my way) project.  My view (like yours) is that the payback is better with an efficient facade than with geothermal.

It is unlikely that I will go PV unless the economics change greatly, but am strongly considering solar hot water (which I have had before) or heat pump water heating.  I am anxious to see what the state of the art is in 2 or 3 years.

Please keep us posted on what you decide.

Bruce

www.pvtsolar.com

The reason why I reference this site is because they have technology ( easily adapted ) where you can still take care of your hot water needs and also provide pv for the house.

One this to remember is that solar is the ONLY thing that pays for itself as long as it is running. Good luck!