Passive solar still amazes me... it was in the 40s last night outside and a high of 59 this afternoon, but the house ranged from 72 to 77 with no active heating systems engaged. When I built my house, I added several redundant heating systems. Based on my research, my passive solar design should provide most of my heating requirements, but I never fully trusted that it would perform according to theory. But so far, it has. In my zone 6a environment, with the given insolation and insulation, passive solar was predicted to supply all of the BTUs needed but for four months of the year -- Dec through March (mostly Jan & Feb) -- when some supplementary heating would be required. Accounting for varying weather from year to year, that prediction has been pretty good. We'll still light up a fire for the occasional Halloween snow storm or April freeze to help warm the bones, but on average, our wood and propane usage is negligible outside of those four months. But on a cold September night, it's still an amazing thing to behold a warm house heated by nothing but solar.