Yes Wanderer, very eloquent. (My posts never are.). I'm questioning part of your post though.
"So instead of say 60 they set it at 40. This has a greater effect on warm winters, because if outside weather is 40 then heater never starts. Remember heat used is not a linear function of difference between outside temperature and thermostat setting temperature due to insulation effect of the house. Therefore if thermostat is 40 and outside is 0 you wont use twice as much as you would if outside was 20."
I beg to differ on this. Heat loss (and the energy required to compensate) is based on the delta T. Let's say everybody has their thermostats set at 60 (40 is 8 above water freezing), and the outside temperature is 10 degrees lower. It will take "X" BTU/Hr to heat. If the temperature is 20 degrees lower it will take "2X" BTU/Hr to heat, and 40 degrees will require "4X" BTU/Hr. Yes, the heat used is linear and does double, then double again.
A drop of average temperatures from 60's to 50's would take us to the first stage, a drop to the 40's would double the consumption, it would take a drop to the 20's to double it again.
I'm loving the green!