What air conditioning power should you choose? Understanding the calculation method

For your air conditioner to provide all the comfort you expect, without increasing your electricity bill, it is essential to choose a unit with power suited to your needs. BTU, floor area, room volume, air conditioning system type and technologies all play a role in calculating air conditioning power. Discover everything you need to know.

Calculating your air conditioning power: why proper sizing matters

Even if you choose a high energy-efficiency, high-performance and high-quality unit, incorrectly estimating the power required for your home can lead to disappointment. If the cooling power is too low, you will need to run your air conditioning system at maximum or near-maximum capacity.

Operating continuously at such high intensity leads to excessive energy consumption and has a significant impact on your electricity bill. In addition, the air conditioner wears out more quickly, especially the motor, which overheats while trying to maintain the temperature you selected.

On the other hand, oversizing your air conditioner is not a solution either. When a unit constantly runs at low speed, the system is more likely to become clogged. Performance also becomes less stable because the unit repeatedly cycles on and off, directly affecting your comfort.

In addition, you will ultimately have paid more for both the unit and installation when purchasing your air conditioning system, without benefiting from any additional performance. Calculating air conditioning power as accurately as possible to combine efficiency, durability and energy savings is therefore one of the first key air conditioning tips to follow.

The criteria that determine the right air conditioning power

When it comes to power, the calculation must include several key factors: the volume to be cooled, the quality of thermal insulation and heat gains. Here is a detailed overview of the criteria to review when choosing an air conditioner with the right power output.

The surface area to cool

To accurately estimate the power required, several factors must be taken into account, starting with the volume you want to cool. Calculating the area to cool is simple: multiply the floor area by the ceiling height.

How do insulation and heat loss affect the calculation?

Volume is only the first step, because the quality of your home’s thermal insulation also plays a decisive role in choosing the power of your air conditioner. The better the insulation, the longer the cool air produced by the unit stays indoors. A well-insulated home therefore requires a less powerful air conditioner to maintain a comfortable temperature.

Conversely, the poorer the building insulation (thermal bridges, single-glazed windows, etc.), the greater the heat loss. Your unit will therefore need more power to compensate for cool air losses and maintain a stable temperature.

Logically, a BBC-standard home (“Bâtiment Basse Consommation” — French low-energy building standard) will require a far less powerful air conditioner than an older house with similar floor area and exposure.

It is also worth noting that glazed surfaces, whether large bay windows or standard windows, are major points of energy loss. However, their orientation changes their impact: a north-facing window with little direct sunlight will require less effort from the air conditioner.

Here is a summary table of the main sources of heat loss in a home and the ways to reduce the power requirements of your air conditioner.

The configuration of your rooms

You must also take into account the layout of the room or home you want to cool. For example, you may calculate 50 m², but if a half-wall or large bookcase actually blocks airflow, your air conditioning power calculation will be inaccurate.

The location of your indoor unit(s)

The location of the indoor unit also affects performance: installing a wall-mounted air conditioner in a high and central position promotes better air distribution than a low installation or placement in a corner, which can slightly reduce the required power.

Other electrical appliances in use

Finally, electrical appliances in operation also contribute to warming the ambient air. If many are present in the room where you install your air conditioner, you must remember to take them into account.

Calling in a professional to carry out a thermal assessment allows all these factors to be combined and helps determine the ideal power for your future air conditioner.

What is BTU, the power unit used for air conditioners?

In France, the power of an electrical appliance is generally expressed in kW (kilowatts). However, air conditioners are more commonly rated in BTU, an English unit meaning “British Thermal Unit”. This unit measures the amount of heat required to cool or heat a volume of air.

How do you calculate the required BTU power for your room?

To obtain an initial estimate of the appropriate power for your air conditioner, use the following calculation: room volume (in m³) × 100. Then add 1,000 BTU for each glazed surface in the room. To convert the result into kW, simply divide the total by 3,415.

A practical example

Now that we know the theoretical calculation, let’s look at an example. You want to calculate the power required to cool a 20 m² room with a ceiling height of 2.5 m and 2 windows, giving a total volume of 50 m³.

Power (kW)=[50×100+(2×1000)]3415≈2.05 kW\text{Power (kW)} = \frac{[50 \times 100 + (2 \times 1000)]}{3415} \approx 2.05\,\text{kW}Power (kW)=3415[50×100+(2×1000)]​≈2.05kW

Based on this result, the ideal air conditioning power would be 7,000 BTU (or 2.05 kW). This can reasonably be rounded up to 2.5 kW for an additional safety margin.

What power should you choose per square metre?

Now that we know the theoretical calculation, let’s take another example to determine the power required to cool a standard 30 m² room with a ceiling height of 2.5 metres:

30×2.5=75 m375×100=7500+2000=950095003415≈2.78 kW30 \times 2.5 = 75\,m^3 \\ 75 \times 100 = 7500 + 2000 = 9500 \\ \frac{9500}{3415} \approx 2.78\,\text{kW}30×2.5=75m375×100=7500+2000=950034159500​≈2.78kW

To cool this 30 m² room, you will therefore need an air conditioner with a power output of around 9,500 BTU, or 2,780 W. This figure can then be rounded up to 3,000 W to provide a little extra margin.

To make things easier, here are some additional examples showing the relationship between appliance power and the surface area to be covered. Again, for a standard ceiling height of 2.5 m, the following power levels apply:

• for a BBC-standard home (“Bâtiment Basse Consommation” low-energy building): 65 W/m²;

• for a home meeting RT2012 standards (French thermal regulation standard): 75 W/m²;

• for a non-renovated house more than 10 years old: 125 W/m²;

• for a house less than 10 years old: 100 W/m².

The following BTU-to-kW conversion guide also helps better understand the available flexibility when choosing the right power:

• 6,000 to 9,000 BTU, or 1,750 W, for up to 25 m²;

• 12,000 to 18,000 BTU, or 3,500 to 5,250 W, for 25 to 50 m²;

• 24,000 BTU, or 7,000 W, for 50 to 70 m²;

• at least 30,000 BTU, or 8,800 W, for more than 70 m² (large spaces generally rely on several indoor units through a multisplit air conditioning system).

Here is a summary table of the BTU and kW power required for different room and property types (according to professional standards and recommendations).

You now have a solid basis for calculating your air conditioning power, which you can refine according to the criteria discussed earlier: electrical appliances, room layout, and more.

What are the differences between a monosplit and a multisplit air conditioner?

A monosplit system includes only one indoor unit that distributes cool air from a single point, while a multisplit air conditioner connects several indoor units to a single outdoor unit. This second system is therefore particularly recommended if you plan to cool your entire home, or at least several rooms. In this case, the total power required, calculated beforehand, is distributed among the different indoor units. Choosing a particularly powerful monosplit system will not cool an entire house from a single air distribution point.

If you choose a reversible split-system air conditioner and live in a region where winters are harsh, allow for 15% additional power for the outdoor unit. Negative temperatures tend to reduce its performance. Indoor units, meanwhile, should be sized according to each individual room.