Graham Sattler,
All the books in all the schools will tell you that the heat gained in a building through the walls is a very small portion of the heat gain.
All of the simulation programs assume that the attic is vented and insulated either at the rafters or just above the ceiling. This is supposed to reduce or stop the heat transfer into the living area.
In the real world, the temperature in the attic climbs to 140 degrees in the summer and stays at the lowest nighttime temp for most of the day. The insulation is vary rarely install correctly and deteriorates over time. The duct work is rarely sealed and leaks badly and is not insulated correctly. All of which drive your electric bill upwards.
I live in Florida in a 2000 sq ft 3/2 house with 9' ceiling with my wife and three cats. We have a hot tub that runs 24/7 ; we have three instant on water heaters; the HVAC is a seer 19 two speed comp0ressor and air handler; The shell is ICF; the roof is metal in a pale yellow and the attic is sealed completely with closed cell foam, the baths and the kitchen vent over the stove are vented to the outside, the windows are dual pane impact with Southern Low E coatings; the South wall does not have any windows. The double front door is Fiberglass and glass and is the only infiltration that I have.
The temp in my attic varies between 78* in the winter to 85* in the summer. My electric consumption runs from 600 kwh to 800 kwh per month with a dollar amount of $100 to $125. My neighbors bills run $300+.
I tried to model the house in EQ and had terrible problems trying to get close to an actual simulation. I cut the infiltration to zero, put R 64 under the rafters and R64 over the ceilings, jacked the SEER to 32 and cut the lighting to .3 w/ft and the CFM to damn near zero and finally got the simulation to come close to actual.
Not that this is going to help you very much, just wanted you to know you are not alone.
Regards
Joe R
LEED AP