We are aggressively setting the temperature back, so that might be it. The morning/evening temps are 68*, during the day is 65*, and night is 58*. I don't believe we are very well insulated (if nothing else, double-hung windows with no weatherstripping and uninsulated sash pockets). Does it make sense that the setback has that much of an effect, or am I working from bad inputs?
In any case, I'll definitely have a heat load calculation performed. I would like to know if my "So here's what I worked out..." has any basis in reality, though.