> How would you even dimension your HVAC if you don't know the numbers?

Well the people setting up the HVAC definitely ran the numbers accounting for windows, insulation, etc. The resulting system does successfully heat/cool the house.

What I'm a little unclear on is whether it was always expected to use this much power in the process. I've been struggling to get info out of my HVAC contractor, may need to hire someone else to come in and look at it.