>>Our utility bills were basically nonexistent, in the spring and fall we might pay $50/month total, and in the dead of a real cold winter or a broiling August we sometimes broke $100 and were really annoyed.<<
Those two sentences contradict each other. I consider $100 a month for a small place, even in the dead of winter, to be a lot. My condo is over 1000 sq ft and the bill is around $60 in the dead of winter.
We routinely get temps over 100 in the summer (with high humidity, which reduces the efficiency of the cooling units) and below freezing in the winter, with occasional bouts of temps below 10 degrees. We had 11 degrees or lower for over a week this last winter.
Also, a condo has firewalls (high R-value) and is insulated by other units on two sides unless you have an end unit. This house has all the living space on one floor, which means more s.f. in contact with the outside world. And in August in Atlanta, that's HOT!
I would happily trade my current August electric bill in a conventional 2-story house for my August bill for all utilities in the solar house.
$100 is a guess, by the way, and I didn't want to exaggerate so I guessed a bit high. I do NOT keep 15 year old utility bills for a house I no longer own!