Sizing an air conditioner in kW for a European home

Updated August 24, 2026 · Claude AC · 2 sources cited

Europe buys air conditioners in kilowatts but shops for them under names inherited from the American market: the « 9 000 BTU » unit on the shelf is a 2.6 kW machine, « 12 000 BTU » is 3.5 kW, « 18 000 BTU » is 5.2 kW. Behind the labels sits one question — how much heat does the room actually gain on a hot afternoon, and what capacity removes it without the machine spending the summer at its least efficient operating point?

The rule of thumb, honestly stated

For a standard European dwelling with 2.5 m ceilings, cooling load per square metre falls in a narrow band, and picking the right band matters more than the arithmetic:

• 70 W/m² — recent or renovated construction, insulated, external shutters or blinds, north or east exposure, not under the roof.
• 100 W/m² — the ordinary case: average insulation, normal glazing, one sunny face. Use this when in doubt.
• 130 W/m² — top floor under a poorly insulated roof, large unshaded west or south glazing, or a Mediterranean location with long hot spells.

Room by room

Readers working in square feet — or comparing against American equipment — will find the same logic in imperial units on our BTU calculator, one page per room size.

The corrections that matter in Europe

European housing stock defeats a pure W/m² rule in four recurring ways, and each deserves an explicit adjustment rather than a vague safety margin.

Ceiling height

Nineteenth-century flats in Paris, Vienna, Budapest or Lisbon routinely have 3.2 to 4 m ceilings. Cooling load follows volume more than floor area: scale the figure by the ratio of your ceiling height to 2.5 m. A 25 m² room with 3.5 m ceilings behaves like a 35 m² room.

Glazing and orientation

An unshaded west-facing window is the single largest gain in most European rooms — a square metre of glass in full afternoon sun admits several hundred watts. External shading changes the class you should pick, and it is why the same room can be a 70 W/m² case with shutters closed and a 130 W/m² case without them.

Occupancy and equipment

Count roughly 100 W of sensible heat per adult present, plus the real draw of anything that runs: a home office with two screens and a workstation adds a few hundred watts all afternoon, and an open kitchen is in a category of its own.

The heating side

If the unit is reversible — and in most of Europe it should be — the heating load often exceeds the cooling load. In Poland, Czechia, the Baltics or continental France, sizing on the winter requirement and checking that the cooling capacity is not absurdly large is the right order of operations. Mediterranean buyers size on cooling. Either way, the energy label gives you both a SEER and a zone-specific SCOP; read the line that matches the season doing the work.

Why oversizing is the more expensive mistake

The instinct is to buy « a bit more, just in case ». It backfires three ways. An inverter modulates down to roughly 30–40 % of nameplate, so a unit twice as large as the load spends the entire season bumping against its minimum, cycling instead of modulating — the operating point where SEER is worst. Second, short runs remove sensible heat quickly and never run long enough to condense moisture out of the air, which is how an oversized machine leaves a room cold and clammy at once. Third, bigger machines cost more, weigh more, and their outdoor units are louder — the number your neighbours and, in several countries, your municipality actually care about, as covered in our guide on AC noise rules.

Undersizing has its own failure mode, less common but real: on the days that matter, a unit below the load runs continuously at full output, never reaches setpoint, and delivers its worst efficiency exactly when electricity is most expensive. The remedy is not a margin on the nameplate but an honest choice of W/m² class — that is where the safety factor belongs.

Multi-splits and whole-home cases

An outdoor unit serving several indoor heads is normally sized below the sum of those heads, because rooms rarely peak together — a diversity factor of 70–80 % of the total is standard practice, and a competent installer will state theirs. Where all rooms genuinely peak at once, the diversity assumption fails and the outdoor unit must be sized closer to the full sum. Equipment options and their trade-offs are laid out on our split AC and heat pump pages.

When to stop estimating

The W/m² method is a specification check, not an engineering calculation. Above roughly 8–10 kW, for buildings with unusual glazing, or wherever a subsidy or building permit is involved, ask for a room-by-room load calculation to a recognised method — the ASHRAE fundamentals approach or the European standards used by design offices. Its real value is not the final kilowatt figure but the itemised list of gains it produces, which tells you which €500 of shading would have removed the need for €1 500 of extra capacity.

Frequently asked questions

How many kW do I need for a 20 m² bedroom?

Between 1.4 and 2.6 kW depending on insulation, floor and exposure — in practice a 2.5 kW unit (9 000 BTU) suits the great majority of European bedrooms, and 3.5 kW is already oversized for a shaded room on a middle floor.

Is 9 000 BTU exactly the same as 2.5 kW?

Close enough to shop with: 1 kW equals 3 412 BTU/h, so 9 000 BTU/h is 2.64 kW and 12 000 BTU/h is 3.52 kW. Manufacturers round the commercial names; the datasheet gives the exact rated capacity, plus the minimum and maximum of the modulation range — that last pair matters more than the headline.

Should I size for the hottest day ever recorded?

No. Sizing for a once-a-decade extreme guarantees an oversized machine for the other 99 % of hours. Size for a normal hot spell, accept a degree or two of drift during a record heatwave, and add shading and night ventilation for the tail — the pattern is visible in the heat records country by country across the European market pages.

Does a portable need the same capacity as a split?

It needs more, and it will still underperform: the single-duct exhaust depressurises the room and draws hot outside air in through every gap, so part of the nameplate capacity is spent cooling the air the machine itself imported. Compare like for like using the label, not the front-panel number.

Can one unit cool several rooms?

Only with open circulation, and never well. Cold air does not turn corners: a single head in a hallway cools the hallway. Doors that stay open, a matched airflow path and realistic expectations are the minimum; otherwise a multi-split with one head per room is the honest answer.

Read next

• Air conditioning in a bedroom: cooling designed around sleep
• The room under the roof: cooling a loft conversion in Europe
• Air conditioning that has stopped cooling: the European diagnosis order
• Heat and health in Europe: how cool a home needs to be, and for whom
• What it costs to install air conditioning in Europe

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