In a modern well-insulated house, glazing is routinely the largest single component of the cooling load, and the four elevations do not contribute equally.
Why orientation dominates
West is the worst. Afternoon sun arrives at a low angle, so it passes almost straight through the glass rather than being partly reflected, and it arrives at the hottest part of the day when the house has already accumulated heat.
East admits similar total energy but in the morning, when outdoor temperature is lower and the house has cooled overnight. Its contribution to the peak load is much smaller.
South takes high-angle sun in summer, which overhangs shade effectively, and low-angle sun in winter, which is useful.
North admits diffuse light and very little direct gain.
A load calculation treats these separately for exactly this reason, and a calculation that lumps all glazing together has thrown away the most important distinction in it.
SHGC, not R-value
Two properties describe a window:
U-factor measures conductive heat flow, and it is what matters for heating.
SHGC — solar heat gain coefficient — is the fraction of solar energy that gets through, from 0 to 1. It is what matters for cooling.
A window can be well insulated and still admit a great deal of solar heat. In cooling-dominated climates SHGC is the number to look at first, and a low-SHGC low-E coating is the single most effective glazing choice.
Typical values run from around 0.7 for old clear double glazing down to 0.25 or lower for modern low-E products.
What reduces it, in order of effectiveness
External shading — an awning, a shutter, an overhang, a tree. Stopping the sun before it reaches the glass is far more effective than anything applied inside, because once the energy is through the glass it is already in the house.
Low-SHGC glazing, on replacement or on a new build.
Applied window film, which is a reasonable retrofit and reduces visible light along with the heat.
Interior blinds and curtains, which help and help least, for the reason above. Light-coloured and reflective backings do better than dark ones.
Overhangs and why they work on the south only
A horizontal overhang shades high summer sun and admits low winter sun, which is close to ideal — on a south elevation.
On east and west it does almost nothing, because the sun is low in the sky when it strikes those faces. West glass needs vertical shading, or something between the sun and the window, or lower SHGC.
This is why a house with a deep porch on the south and unshaded west glass can still have an afternoon problem.
Room-by-room consequences
The room with west glass will be the hot room in the afternoon, and it will not be fixed by more capacity at the equipment.
It needs its share of the airflow, which is what the room-by-room part of a load calculation is for. A house where the ducts were sized by rule of thumb will typically under-serve exactly that room.
What this changes in practice
On a new build or a remodel, glazing area and orientation are design decisions with a direct equipment-size consequence. Reducing west glass or specifying low-SHGC for it can move the calculation by half a ton.
On an existing house, external shading on the west elevation is one of the cheapest available reductions in cooling load, and it is frequently overlooked in favour of equipment changes that cost many times more.
Before sizing, any planned window replacement should be reflected in the calculation, since new glazing can lower the load enough to change the equipment selection.