Heat loss is calculated by knowing the R rating of walls, ceiling, windows, doors, loss through ground, not to mention air infiltration. You also have to know the delta temp between inside and outside temps. Windows are rated in U ratings which you convert to R. R = 1/U Oh, you also need to know the surface area of each part of the building too, doors, windows, walls, ceiling, floor.
Give me that, the rest is very simple.
It's very cheap to heat just air.