We have passive solar, bermed home with tri-pane, high solar heat gain windows. Heat load is ~14,000 (not really sure, we have lots of thermal mass so that makes the real-world calculations tough.) we will be using a wood stove and have decided to get an electric mini-boiler (7Kw) as heat source for back-up radiant in-floor (we are going all electric and can use off peak energy rates plus plan to do solar PV in the future).  We are going to live in the house for a season or two to see what the load really is.  Meanwhile, what to do about hot water.  Solar DHW is way too expensive to put in for a 2 person adult household with no dishwasher and low HW use.  We are wondering if the 7kw mini-boiler could also be used to heat DHW (with indirect water tank) or will that be really inefficient to run, especially in the summer? Should we just get a small water heater and keep them separate for now?

Also wondering if electric heat pump water heater would be worth the $$ for us in Wisconsin climate.  We would put in basement which is about 20 feet into the earth due to berming, so stays at 50 to 60 degrees.  Does this low temperature drop its efficiency?