A south-facing room in the UK collects sun for much of the day, so the cooling load is higher than a same-size room on the north. Glass area, shading and thermal mass decide how far above. Survey it. External blinds, solar-control glass or even a deep reveal can cut the kW you need. The air conditioner then holds the room with the gains that remain. Do not buy a bigger unit and ignore the glass.
Midday sun on south glass
Midday and early afternoon sun on south glazing is the driver. A bay, a set of patio doors or a rooflight multiplies it. Heavy brick can delay the peak, so the room is still hot at 6pm when the chart said the sun had moved on. Lightweight extensions peak faster and harder. Expect the peak in the afternoon and early evening.
Two bedrooms, one floor area
A floor-area rule misses this completely. Two 14 m2 bedrooms, one south with a bay and one north with a small sash, are different jobs.
Shade you will actually use
Thin internal blinds help less than external shading or solar-control glass, because the heat is already inside the glass line. Tell the installer what you will actually use.
- Measure glass, not only floor area.
- Note outside shade from trees, bays and neighbouring houses.
- External shading reduces plant size.
Calculate, do not jump a size for the compass
Many south-facing living rooms land on 3.5 kW or more after a proper look. Bedrooms can still be 2.5 kW if the window is modest and the door stays shut. The point is to calculate, not to jump a size because the compass says south. An inverter split modulates. It still needs enough capacity for the peak hour. A unit that runs at full speed all afternoon is undersized, loud and less efficient than the label suggested.
Off the glass, clear of the curtains
Keep the chassis off the glass if you can, so it recirculates room air rather than sensing a hot pocket at the window and misreading the space. Do not put it where curtains will be drawn across the intake. Usually put it on a solid wall where the air can circulate and curtains cannot choke it. Above the south window is often the wrong guess. Heating mode still matters on a south room in winter once the sun has gone. A reverse-cycle air-to-air heat pump covers both. Read the heating table if the radiator is small.
A coil that is not boxed into a sun trap
The outdoor unit also prefers not to sit in a sun trap with poor airflow, against a south wall with no clearance. Leave the coil's intake and discharge as the manufacturer shows. F-gas commissioning applies. In England, Planning Portal conditions cover the outdoor heat pump. A south garden boundary can be the neighbour's sitting-out space. Put noise in the siting decision.
Frequently asked questions
Does every south-facing room need a larger air conditioner?
South glazing raises the load compared with a shaded room of the same size. How much larger depends on the glass and the shade. Survey it rather than adding a fixed percentage.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.