There are so many factors involved in the amount of supplemental (or central) heat necessary for any passive solar home - especially one of bermed construction that I wouldn't even consider trying to recommend any. However, here are my thoughts after being in our house for a year.
1 - Design We opted for a modified design (usually for full passive value, you would build in modules with all rooms open to the sun and each other). You can see from our design that we placed baths and laundry behind a hallway as a personal preference. The shower bath therefore is the coolest location in the house and I like to supplement it for those early morning showers.
2 - Building style - insulation, amount of thermal mass concrete, amount of windows, sides bermed, preparation of the slab (see previous posts during construction).
All of these add to the retained heat .
3 - Window treatments - we do not yet have any insulated window treatments to withhold the heat during the night. We may add these later.
4 - Resale - it can be very difficult to get a mortgage for a house without central heat. We never use our radiant heating system though and at times feel it was wasted money.
5 - Type of wood stove. We chose a Tulikivi (because I fell in love at first site). It gives a slow radiant heat - no major hot spots and is great for our living area. We could have also installed a small wood stove in the back of the house and probably only supplemented with wood.
So, in our situation, we do need our Tulikivi. Only for the times that we have more than one day without sun. If during those dreary weeks we wish to have the bedroom and shower room above 64 or so, we also need additional heat at the west side of the house. Do we need or use the radiant heating system - no.