How sun exposure affects air conditioner size
An unshaded west window can add more heat than three people.
A west-facing window contributes up to 150 W per square metre of glass, against 60 W if it faces north or sits in shade. Blinds or curtains cut that by about 30 %, and external shading by more than half.
Solar gain by orientation and shading
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| Orientation | Unshaded | Blinds or curtains | Awning or shade |
|---|---|---|---|
| North or permanently shaded | 60 W/m² | 42 W/m² | 27 W/m² |
| South | 110 W/m² | 77 W/m² | 50 W/m² |
| East or west | 150 W/m² | 105 W/m² | 68 W/m² |
Why west beats south for heat
It is counterintuitive, but in summer east and west windows take more load than south-facing ones. The reason is the sun's altitude: in summer the midday sun is very high, so it strikes a south-facing window at a glancing angle and much of it reflects away. Morning and evening, by contrast, the sun is low and comes in almost perpendicular through east and west glazing.
West is the worse of the two, because it coincides with the hottest accumulated part of the day: the house is already warm when that sun arrives.
How much it weighs in the calculation
In a 200 sq ft room with 32 sq ft of glazing:
- North facing with blinds: 126 W, about 5 % of the total load.
- West facing unshaded: 450 W, nearly 17 % of the total load.
Those 324 W of difference are equivalent to almost three more people in the room.
External shading always wins
An interior curtain stops heat after the sun has passed through the glass: the radiation is already inside and turns into heat in the room. An awning, external shutter or tree stops the radiation before it enters, which is why they are far more effective even though they look like the same idea.
What to do in practice
- Close blinds before the sun hits the window, not once the room is already hot.
- Prioritise external shading on west-facing windows, which contribute most.
- Ventilate at night once the outside temperature drops below the inside one.
- Do not obsess over north-facing glass: it is rarely the problem.
Sources and methodology
Conversions use exact constants: 1 BTU/h = 0.29307107 W and 1 frigorie/h = 1 kcal/h = 1.163 W. The component sizing model (load per volume by insulation level, 120 W per person, solar gain by glazed area) is set out in full on the air conditioner size calculator. It is a guideline estimate: a professional heat-load calculation considers considerably more variables.
Frequently asked questions
Which window orientation gets hottest in summer?
West, followed by east. They take the low afternoon and morning sun, which enters almost perpendicular to the glass, while the summer sun strikes south windows from high above.
How much heat do blinds block?
In the calculator's model, about 30 %. An awning or permanent shade cuts more than half.
Interior curtains or exterior awning?
Exterior, without question. External shading stops radiation before it passes through the glass; interior shading acts once the heat is already inside.
How much does a window add to the calculation?
In a 200 sq ft room with 32 sq ft of unshaded west-facing glazing, about 450 W, nearly 17 % of the total load.