Choosing · Read 7 min
AC power for 20, 40 or 100 m²: how to size right
By the Aircadia team
Reckon on an average of 100 W/m² for a standard home, but the real range runs from 65 W/m² in a well-insulated new build to 125 W/m² in a poorly insulated older flat. In Nice, add 10 to 20 % for a south-facing exposure, a top floor or a seafront location. A proper heat-load survey remains essential to settle it.
The essentials
- The “100 W/m²” rule is a starting point, not a technical standard.
- 1 kW ≈ 3,412 BTU/h — manufacturers usually think in BTU.
- On the Riviera, always adjust upwards for south exposure, top floor and seafront.
- Both over- and under-sizing are costly (comfort, energy use, lifespan).
How is AC power calculated per m²?
The required cooling capacity depends on six parameters, not just floor area: room volume (ceiling height), insulation level, orientation, size and number of glazed openings, use (bedroom, living room, kitchen area) and climate zone. Two 40 m² living rooms can therefore call for very different capacities.
As a baseline, the following orders of magnitude apply for insulation, on a standard 2.50 m ceiling height:
- New RE2020 build or BBC renovation: about 65 to 75 W/m².
- Standard well-maintained home: about 100 W/m².
- Old poorly insulated flat, top floor under roofing, large glazing: up to 125 W/m² or more.
For conversion: 1 kW = 3,412 BTU/h. A 12,000 BTU split therefore delivers roughly 3.5 kW of cooling.
What power for 20, 40, 60, 80 or 100 m²?
Here are indicative ranges, before adjustments for exposure or floor. Values are given as cooling capacity, the figure shown on the AC unit’s spec sheet.
| Floor area | New / BBC (~70 W/m²) | Standard (~100 W/m²) | Old poorly insulated (~125 W/m²) | BTU equivalent (standard) |
|---|---|---|---|---|
| 20 m² (bedroom, studio) | 1.4 kW | 2.0 kW | 2.5 kW | ~7,000 BTU |
| 40 m² (living room) | 2.8 kW | 4.0 kW | 5.0 kW | ~13,500 BTU |
| 60 m² (2-bedroom flat) | 4.2 kW | 6.0 kW | 7.5 kW | ~20,000 BTU |
| 80 m² (3-bedroom flat) | 5.6 kW | 8.0 kW | 10.0 kW | ~27,000 BTU |
| 100 m² (single-storey house) | 7.0 kW | 10.0 kW | 12.5 kW | ~34,000 BTU |
For a multi-split, you add up needs room by room, then apply a simultaneity coefficient of 0.7 to 0.9 on the outdoor unit — all indoor units never run at full load at the same time. That’s why an 8 kW outdoor unit can feed four indoor units totalling 10 kW.
Why increase the power in Nice and on the coast?
The table above assumes an average room, east- or north-facing, on a mid-level floor. On the Riviera, several factors push the load noticeably higher:
- South or south-west exposure: roughly +10 %, because of intense summer sunshine. On the seafront, reflection off the water amplifies solar gain.
- Top floor under roofing: +10 to 15 %, as a heated roof transfers significant heat. Typical case: studios under the roofs of Old Nice or the Port area.
- Large unshaded glazing: each m² of south-facing glass can bring in up to 600 W at peak. A 40 m² living room with 8 m² of south-facing bay windows easily goes from 4 kW to 5-5.5 kW.
- Kitchen open onto the living room: +500 W to 1 kW to absorb the load from oven, hobs and fridge.
- Number of occupants: each person releases around 100 W at rest, more when active. For a coworking office in Sophia Antipolis or an Airbnb for 4 guests, that’s no longer marginal.
Concrete example: a 25 m² studio under the roof in Nice, south-facing, with unshaded glazing, should not be sized at 2.5 kW as the table suggests — aim for 3.5 kW to avoid the mid-summer failure.
The “100 W/m²” myth: what really matters
The 100 W/m² rule is everywhere online — it’s a convenient shortcut, not a technical standard. The French ADEME explicitly recalls that proper sizing requires a heat-load survey by a qualified professional, factoring in floor area, volume, insulation, orientation, glazing count and climate zone. Multiplying your floor area by 100 W regularly leads to two errors:
- In a recent well-insulated home, you oversize by 30 to 40 %.
- In an old south-facing flat in Nice, you undersize by the same margin.
The reference for reliable sizing is the EN 12831 standard (heat load calculation), used by HVAC engineers and certified installers.
What happens if the AC is wrongly sized?
Both over- and under-sizing cause problems that are often invisible the first year.
| Case | Symptoms | Consequences |
|---|---|---|
| Oversized | Short cycles, cold-and-damp feel, draughts | +20 to 25 % electricity use, premature compressor wear, poor dehumidification |
| Correctly sized | Slow steady operation (Inverter), stable temperature ±0.5 °C | Optimal consumption, 12-15 year lifespan, constant comfort |
| Undersized | Unit runs non-stop, target temperature never reached | Compressor overheating, early failure, bills that explode during heatwaves |
Oversizing, very common, costs more than it saves. Inverter technology limits the problem by modulating capacity, but does not rescue a major sizing error. That’s why Aircadia always starts with a free on-site heat-load survey before any quote.
FAQ — AC sizing
How many BTU for 20 m²?
Reckon on about 7,000 BTU (2 kW of cooling) for a standard 20 m² room in Nice, up to 9,000 BTU if it’s south-facing or on the top floor. A BTU value says nothing about efficiency: at equal capacity, an A+++ Inverter model will consume 30 % less than an A-rated one.
Which AC for 100 m²?
For 100 m², the range runs from 7 kW in a well-insulated new build to 12.5 kW in a poorly insulated older home, with a typical pivot around 10 kW. Over that floor area, a multi-split (3 to 5 indoor units) is almost always more relevant than a single-split that would cool only one room.
Does an oversized AC use more energy?
Yes — contrary to a common belief. An oversized unit runs short on-off cycles that wear out the compressor and raise consumption by 20 to 25 % versus a correctly sized unit. It also fails to dehumidify the room properly, creating an uncomfortable cold-and-damp feel.
Do I really need a professional heat-load survey?
For any fixed installation above €2,000, yes. A heat-load survey avoids both pitfalls — over- and under-sizing — which cost €300 to €800 a year in wasted power and cut several years off the unit’s lifespan. In Nice, Aircadia includes it free of charge in every quote.
Sources and references
- ADEME, How to choose your air conditioning (2026 update, French).
- ADEME, guide Comment garder son logement frais tout l’été ? — product page and free PDF.
- Standard EN 12831 — method for calculating design heat load (reference used by HVAC engineers for sizing).
Want a precise, costed sizing with no strings attached?
Aircadia works in Nice, Cannes, Antibes and across the 06. We run a free on-site heat-load survey, tailored to your exposure, insulation and floor level — and deliver a clear quote within 48 h, with the capacity and technology (single-split or multi-split, reversible, ducted) that really fit your home.