We are building (builder/owner) a 1350 sq. ft home in SW Wisconsin, bermed into the a South facing hill with passive solar design, superinsulated, high performance triple pane windows and high thermal mass (concrete floors and earth block wall). We have an open floor plan on one level with a small basement area for utilities. We put in in-floor radiant in concrete slab.  We believe we won't need to use it very often (especially if we put in a wood stove), but have been getting conflicting info from HVAC on what to do to power the in-floor. With our possible low need, we would rather wait and monitor energy use but code requires us to have a backup before we move in.  We would like to be as carbon neutral as possible and are saving money for PV electric.  We are trying to decide if a regular sized solar DHW system (only 2 adults with high tolerance for dirt) with tankless electric backup would work for us.  Or do what everyone is telling us and put in a huge solar DHW tank with heat sinks and backup high efficiency propane boiler.  We are wary that this will be worth it given the small amount we may need it and reluctance to put in propane when we want to go electric PV..  Any advice or great ideas for us? 

How about something as simple as an in-wall propane direct vent heater? Low cost install, still fairly efficient, reliable back up heat.
 
Partly the decision should be based on costs of energy ie electricity vs propane. I am in roughly the same position you are although I wouldn’t call my house superinsulated.
 I am consuming about 150 gallons/year of propane for heat, cooking and hot water. I have a mod con gas boiler. Realistically it is overkill because it is the third heating system in the house (solar- woodstove-boiler).
 Probably a standard efficiency boiler would have only used 15 gallons more propane per year.
I have a solar DHW system with an 80 gallon tank  that feeds a second indirect 50 gallon tank that is heated by the propane boiler. The boiler has fired once or twice since april, it has been all solar since then. The water in the solar indirect gets up to around 130-140 deg most days in the summer,the aquastat in the second tank is set to 115, so as long as I use 20 or so gallons of hot water per day (which draws new hotter water from the solar tank) the water in the second indirect tank does not cool enough to cause the boiler to fire.
 
My costs for electricity are ~$0.20 kwh so I try to avoid electricity usage as much as possible.
 IF electricity was say $ 0.08 kwh I would have just installed a propane fired wall heater for heat, and hot water would have been a 2 tank system with the first tank an 80 gallon indirect tank that was heated by solar, and a second tank like a 40 gallon electric. The solar would be the preheat in the winter, and provide all the hot water in the summer.

This would be simple cheap and efficient.
 Cheers
 Eric