Which air conditioner for which room size?
In short: As a rule of thumb, a room requires around 60 to 100 watts of cooling capacity per square metre, depending on insulation and sunlight. For 20 m², that is 1.2 to 2.0 kW, i.e. a unit with 9,000 BTU (around 2.6 kW); for 40 m², 2.4 to 4.0 kW, i.e. 9,000 to 18,000 BTU.
Contents
How much cooling capacity is required per square metre?
The correct size of an air conditioner depends on the cooling capacity in kilowatts (kW), not on the room size alone. The room size is merely the starting point; insulation, sunlight, ceiling height and heat within the room all affect the required capacity. This page uses a range rather than a single figure, just as manufacturers do for guidance.
Manufacturers and guides specify a cooling requirement of approximately 60 to 100 watts per square metre for living spaces with standard ceiling heights. Bosch specifies 63 to 94 W/m², whilst Daikin specifies around 0.1 kW per m². Well-insulated new-builds fall at the lower end of the scale, whilst unrenovated older buildings and loft conversions fall at the upper end.
Here’s how to calculate: Area in m² multiplied by 60 to 100 W, divided by 1,000, gives the power requirement in kW. A room measuring 25 m² therefore requires approximately 1.5 to 2.5 kW.
This is a guide value. It does not replace a cooling load calculation in accordance with VDI 2078, nor does it replace the planning carried out by the specialist firm installing the system.
Which power rating is suitable for which room size?
The power class is the smallest for which the rated power approximately meets the demand. Calculations are based on 60 W/m² (well insulated) and 100 W/m² (poorly insulated or sunny), with a ceiling height of up to 2.5 m.
| Room size | Cooling requirements | Performance class |
|---|---|---|
| 15 m² | 0.9 to 1.5 kW | 9,000 BTU |
| 20 m² | 1.2 to 2.0 kW | 9,000 BTU |
| 25 m² | 1.5 to 2.5 kW | 9,000 BTU |
| 30 m² | 1.8 to 3.0 kW | 9,000 BTU to 12,000 BTU |
| 35 m² | 2.1 to 3.5 kW | 9,000 BTU to 12,000 BTU |
| 40 m² | 2.4 to 4.0 kW | 9,000 BTU to 18,000 BTU |
| 50 m² | 3.0 to 5.0 kW | 12,000 BTU to 18,000 BTU |
| 60 m² | 3.6 to 6.0 kW | 18,000 BTU to 24,000 BTU |
| 70 m² | 4.2 to 7.0 kW | 18,000 BTU to 24,000 BTU |
| 80 m² | 4.8 to 8.0 kW | 18,000 BTU to 24,000 BTU and above |
| 90 m² | 5.4 to 9.0 kW | 24,000 BTU and above (floor-standing unit or multi-split) |
A single wall-mounted unit is no longer sufficient for outputs above around 7 kW. For higher outputs, the range includes a ceiling cassette unit with 42,000 BTU (12.3 kW) and a floor-standing unit with 48,000 BTU (around 13.5 kW). A multi-split air conditioner is suitable for multiple rooms.
For a bedroom of up to about 26 m², the 9,000 BTU class is enough even with a lot of sun; what else matters there, above all a quiet indoor unit, is explained in Air conditioner in the bedroom.
How do you convert BTU to kW?
BTU is the unit in which air conditioners are usually specified. 1 kW corresponds to 3,412 BTU per hour (NIST). The classes are market designations; the rated power of the units differs slightly from these. Our sets specify the following values in the catalogue:
| Performance class | converted | Rated power of the sets in the range |
|---|---|---|
| 9.000 BTU | 2,6 kW | 2,6 kW |
| 12.000 BTU | 3,5 kW | 3.3 to 3.6 kW |
| 18.000 BTU | 5,3 kW | 5.1 to 5.3 kW |
| 24.000 BTU | 7,0 kW | 6.6 to 7.0 kW |
Manufacturers also specify their own room sizes for their units. For the sets in the range, these are, depending on the set, 16 to 30 m² for 9,000 BTU, 22 to 45 m² for 12,000 BTU, 33 to 60 m² for 18,000 BTU and 45 to 90 m² for 24,000 BTU. These figures are lower than the rule of thumb given above. In case of doubt, refer to the specifications in the individual model’s data sheet.
What increases demand?
- Sun: According to Bosch and Daikin, large windows facing south or west increase energy demand by 10 to 20 per cent.
- Top floor: It gets significantly warmer under the roof in summer; use the upper value of the range.
- Ceiling height: According to Daikin, the requirement increases roughly in proportion to the height above approximately 2.5 m.
- People and appliances: According to Bosch, each person emits around 80 to 120 W of heat, whilst a PC, television or hob emits its power consumption. In the kitchen, demand is 10 to 20 per cent higher, according to Bosch and Daikin.
Would you prefer to buy a larger one?
Not much larger. A significantly oversized unit reaches the target temperature quickly, may switch off and on more frequently, and consequently performs less effective dehumidification. Choose the class that covers the upper limit of your range, and no more than one class above that.
For multiple rooms, calculate each room individually. Instead of several individual units, a multi-split air conditioner with one outdoor unit can supply several indoor units. The arguments for each option are set out under ‘Air conditioner for multiple rooms’.
How much does an air conditioner cost per performance class?
As a split-set comprising an indoor and outdoor unit, an air conditioner in the range costs about 500 to 850 € for 9,000 BTU, about 500 to 1,400 € for 12,000 BTU, about 850 to 1,750 € for 18,000 BTU and about 950 to 1,700 € (incl. VAT, as of October 2026) for 24,000 BTU. Installation by a certified specialist contractor is not included in this price. The price on the product page is decisive.
The sets for each class are listed under split air conditioners with 9,000 BTU, split air conditioners with 12,000 BTU, split air conditioners with 18,000 BTU and split air conditioners with 24,000 BTU. The installation and commissioning of a split air conditioner should be carried out by a certified specialist company; details of how this is done can be found under ‘Professional installation of your air conditioner’.
Split air conditioners with 9,000 BTU
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