Just make it a big 'un, and extremely well insulated if you're gonna try & hack solar onto an indirect. Standby losses of tanks do not increase linearly with size, but bigger tanks DO translate into lower-temp/higher collection efficiency operation of the solar collectors, increasing your shoulder-season solar performance immensely.    

But again right-sizing the tank will be different if the DHW is being operated as a separately zoned indirect, vs a unitary buffer tank combi w/heat exchanger for the DHW.  There are too many particulars to get right before determining the most-optimal topology of the thing as a whole, but a buffer-centric approach where all heat sources feed into, and all heat loads draw from one tank, maintained at a minimum DHW temp (but allowed to run higher when the solar is available) can work well (even with condensing boilers & slabs, with a bit o' plumbing & control, taking the boiler return directly from the slab rather than the bottom of an un-stratified tank during the heating season.) 

In most systems the DHW fuel use will be 1/4 or less of the total, but in a mini/micro-heat load of something approaching a PassiveHouse, DHW will dominate both annual fuel use and peak loads.  At some point it's all DHW, and the rest of it becomes irrelevant.  (It would be easier to assess with actual numbers...) 

I DO like the PassiveHouse approach on new construction- but with loads that low radiant starts to seem a bit... rococo(?), more complexity than can be rationalized, with little of the extra under-foot cush factor of warm floors since the temps are so low.

OK, I am pretty sure Rube Goldberg did not experiment with drain back systems, but for solar I think the drain back though not as trendy as evacuated tube is the simple bullet proof way to go. To hot, no heat transfer (no over heat), to cold no heat transfer, (no freeze). My house has 2 large evacuated tube systems feeding a 120 gallon tank, if I leave town my tank can get pretty darn hot if I forget to switch the control to vacation, in that mode the pump runs to dump heat in the night air.
one time I had a pump fail, she got really hot in the panel +245 that is hot hot hot, the tubes last 15 years, flat panels 25,
the Drain down is simple, the old ones from 25 plus years back seem to be going strong.
I like the simplicity of the drain back.
These are easily adapted to a domestic electric tank, a pre-feed for a existing system, and sure you can use a nice duel coil 89 gallon SS tank with some type of boiler back up.
But don't through out the drain back with the wash,
Dan

Dan,

uncertain fuel costs are up to my clients to value appropriately as they see fit, not us. I can do the math. They can decide what they want to plan for. I can say for any fuel cost we have ever seen, in heating dominated climates, solar heating is not cost effective unless it allows you to drop a heating system nearly entirely which is only possible in the tiniest of heat loads. Solar DHW, different story. Solar heating in the south, different story. But that's the story so far.

Rob,
right again,
I was agreeing with Morgans combi core though those are costly like many other things expanding the idea to coupling with a solar drain down. The combi would make the storage for the drain down, and handily make the heat on a mid size home. So if there was a surplus for free solar heat on that of not Grey day in winter in cold town or free domestic H20 in summer, this might be an ok alternative. These systems intrigue me, I would like to be offering a design alternative with every system I sell.
We as a society have been grossly misled related to energy. It is these types of changes from a design forum of what has and hasn't or might partially work that need to be known, kicked and tried.
I think that is what is good going on here is that there is a solid debate and honesty of what is and is not working from a lot of talented mechanics/designers.
Dan

OP chiming in again...

Well, I've learned a lot. But not enough. I'll wait for the final plans and energy modeling etc. I'll probably end up with a big gas water heater from Home Depot with an external plate heat exchanger. I'm guessing I could do it for under $1000. A Polaris or similar would be nice, but are they worth thousands more? How much more life can you get out of them?

Thanks,
-m

pbrane, Where are you building this house? How many occupants will there be? Are you likely to be using very many heat generating appliances such as cooktops, ovens, TVs, computers, etc. The reason I ask is because with a small energy efficient house the actual heating load your heating system will see may be pretty small, so small in fact that in spite of all the great benefits of radiant heat it may not be practical. You may be looking at a lot more cooling load than you think you will have. As far as heating it's quite possible, like Dana says, that water heating will be more of an issue than space heating.

Radiant heating with all its piping, heat panels, controls, etc., in a low heat load home is kind of like using a Mack truck to haul sand for your kid's sandbox. It can be done, but it may not be smart.

I built a 2000 sf ICF house. I really wanted to use radiant heat, but the heat load calcs were so low I couldn't justify the cost. And, I was quite surprised at just how much heat is generated by just living in the house, and that's with only 2 adults and 2 dogs! Add to that solar gain in the spring and fall and I swear there's times I run the a/c while my neighbors run their furnace!!! Radiant heat is just about the best heating method there is. It just isn't always the smartest choice, financially.

high efficiency heat sources are worth extra money depending on how much heat they will need to make: that is answered with heat loss, DHW usage, and degree day calculations.

I follow with interest you guys.  However I don't know what MOd cons and combi cors are!  Can you explain?   Regards  baron

I think you hit it on the head with the "Home Depot" thing.

I fold.

modcons are "Modulating condensing boilers". pretty much the cream of the crop in the gas fired world.

Combicor is a particular water heater made by Bradford white with a coil in it, to allow for a heating system to draw water from it without an external heat exchanger.