Why is electric heat capacity calculated using floor area instead of space volume?
Floor area is used for two reasons:


The temperature of a room results from a balance between the rate at which heat enters and the rate at which it's lost through the walls.  The rate of loss, in turn, is proportional to the surface area of the walls, floor, and ceiling, which, for a normal-shaped building, is roughly in proportion to the floor area.
Floor area is easier to measure.