What air conditioner capacity for a room or a flat? Why "watts per m²" is not enough
The EcoClimo team
5 Nov 2026 - 04 min read
It is often the first question asked before an air conditioning project: what capacity do I need for this bedroom, this living room, this flat? People look for a simple figure, and online charts are happy to supply one, calculated from floor area alone.
That shortcut gives an order of magnitude, but it often leads to the wrong choice. Two rooms of the same size can have very different needs. Here is what really determines the capacity required, why an overpowered unit is a poor bet and what a site survey adds.
The watts-per-m² rule: a starting point, not an answer
You often read that you should allow around a hundred watts of cooling capacity per square metre, for a standard ceiling height of about 2.50 m. By this rule, a 12 m² bedroom would need about 1.2 kW and a 30 m² living room about 3 kW.
The approach has the merit of simplicity. But it starts from an "average" home that does not exist. It ignores everything that lets heat into the room or produces it there, which is exactly what the air conditioner has to offset.
A useful reminder in passing: an air conditioner's capacity is expressed in kilowatts (kW) or in BTU/h, an imperial unit still common on product sheets. 1 kW is roughly 3,412 BTU/h. This is the cooling capacity, not to be confused with the electrical power drawn, which is much lower.
What changes the real need
| Factor | Effect on the need | Example |
|---|---|---|
| Orientation | Sharp rise facing south and west | Living room with a west-facing picture window |
| Position in the building | Sharp rise under the roof | Top floor, attic bedroom |
| Insulation and glazing | Lower if insulation is good | Recent building, high-performance double glazing |
| Ceiling height | Rises with volume | Haussmann-era flat with ceilings above 3 m |
| Occupants | Rises with the number of people | Living room where you often entertain |
| Equipment | Rises with heat-producing appliances | Home office, open-plan kitchen |
Sun and glazing
This is the factor that weighs most in summer. A west-facing picture window gets the sun just when the outside air is at its hottest. The same room facing north will need considerably less. External protection (shutters, blinds, brise-soleil) makes a big difference, because it stops the heat before it gets in.
Position in the building
A top floor under a poorly insulated roof, especially in a converted loft, builds up heat all afternoon and releases it in the evening. For the same floor area, a flat on an intermediate floor, surrounded by other homes, is far better protected.
Volume and occupants
The per-m² rule assumes a standard ceiling. In an older flat with ceilings above 3 m, the volume of air to treat increases noticeably. The people in the room count too: each gives off, at rest, heat in the region of a hundred watts. Computers, televisions and hobs add to that.
To prepare for a conversation with a technician, measure your rooms, including the height. A laser distance meter makes the job quick and accurate, especially with high ceilings.
Why an oversized air conditioner is not a good idea
When in doubt, it is tempting to go "a bit bigger, to be on the safe side". That is often a mistake.
- It runs in fits and starts. An oversized unit reaches the set point very quickly, then slows right down or stops before starting again. Even with inverter technology, which modulates output, every unit has a minimum below which it cannot go.
- It dehumidifies poorly. Dehumidification happens while the unit runs. Cycles that are too short leave the air cool but clammy, which feels uncomfortable.
- It wears out faster. Repeated starts put strain on the compressor.
- It costs more, to buy and to run, for comfort that is no better.
An undersized unit has the opposite problem: it runs flat out continuously in hot weather and cannot hold the set point during a heatwave. The aim is therefore accuracy, not the biggest possible margin. That is also the logic of a sober installation: a correctly sized unit uses what it needs, and no more.
Reducing the need before choosing the capacity
Before sizing, it is worth asking what can be done to limit heat gains. A few simple measures reduce the need and sometimes allow a smaller unit:
- close shutters and blinds on the sunny side during the day;
- fit external protection to the most exposed windows;
- air the home at night and early in the morning, when the outside air is cooler;
- limit heat-producing appliances during the hottest hours.
Indoors, a thermal blackout curtain also helps in an exposed bedroom, even though it is less effective than external protection.
The role of the site survey
Proper sizing relies on a visit, not a form. During the survey, the technician:
- measures the rooms and notes orientation, glazing and solar protection;
- assesses insulation, the home's position in the building and how each room is used;
- checks possible locations for the indoor units and the outdoor unit, the route for the pipework and condensate drainage;
- takes heating into account if you want a reversible air conditioner, as the winter requirement can be decisive;
- flags the formalities to plan for, such as approval from the shared building (copropriété) for an outdoor unit on the frontage.
For a flat with several rooms, they also weigh up several independent units against a multi-split system, where a single outdoor unit serves several indoor units. In that case, the outdoor unit's capacity does not necessarily equal the sum of the indoor units, since not every room needs maximum cooling at the same time.
A correctly sized and properly installed unit then stays efficient if it is serviced regularly. Our technicians provide that servicing throughout Île-de-France, town by town.
Planning to equip your home or your shop? Our air conditioning installation service always starts with a site survey, and our advice and support helps you compare the options. Describe your project via the contact page to receive a free quote within 48 working hours.
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