What cooling output do I need
What cooling output do I need
One of the most common questions we deal with from customers before buying an air conditioner is simply: "What output do I actually need?" At first glance the answer seems trivial - just look at the room's floor area and assign it kilowatts according to some table. In practice, however, it's more complicated, and this is exactly where most of the mistakes happen that customers later regret - either they buy a unit that can't cool the room to a comfortable temperature even in summer, or, on the other hand, an oversized air conditioner that cools the room in a few minutes but then cycles on and off repeatedly, runs noisier, and leaves the room feeling uncomfortable due to sharp swings in temperature and humidity.
In this article we explain, in practical terms, how air conditioner output is actually calculated, what factors influence it besides the room's floor area, how to avoid the most common estimation mistakes, and, using specific real products, we show which output suits which space. The goal isn't to sell you the largest unit in the range, but to help you choose one that will work efficiently, quietly and economically in that particular room, for many years without major problems.
Why output really matters
Air conditioner output is stated in kilowatts (kW) of cooling, and sometimes heating, capacity. At first glance, when looking at product specs, it's just one number, but in reality it determines almost everything essential about how the unit performs:
- How quickly and how deeply the unit can cool the room at a given outdoor temperature.
- How efficiently (economically) it will operate during normal use, not just at extreme peaks.
- How noisy the unit will be - an undersized air conditioner runs at 100 % fan and compressor output most of the time, which is noisier than a slightly oversized unit running at 40 - 60 %.
- How long the unit will last without excessive wear - frequent short on/off cycles in an oversized air conditioner put more strain on the compressor than smooth, longer operation at medium output.
- What humidity level the room will have - an air conditioner dehumidifies the air while cooling, but with cycles that are too short (oversizing) it doesn't have time to dehumidify sufficiently, so the air may be cool but unpleasantly humid.
This is exactly why it's worth paying a little more attention to calculating output than just a rough "by eye" estimate. Both an undersized and an oversized air conditioner ultimately cost the customer more money - either in the form of higher electricity consumption, or in the form of a shortened compressor service life.
Basic calculation: room area and height
The simplest and most commonly used rule of thumb is based on the room's floor area and a standard ceiling height (2.5 - 2.7 m), which is common in most Slovak flats and family houses. The basic rule is:
Allow roughly 1 - 1.2 kW of cooling output for every 10 m² of room area, at a standard ceiling height of up to 2.7 m and without significant additional heat loads.
This rule is also the basis for the general recommendations you'll find on most air conditioners in our e-shop:
- up to approx. 15 m² (smaller bedroom, home office) - a unit with around 2.5 kW output is sufficient,
- up to approx. 20 - 25 m² (average living room, larger bedroom) - around 3.5 kW is suitable,
- up to approx. 35 - 40 m² (large living room, open-plan kitchen + living room) - the required output is already around 5 kW,
- above 40 - 50 m² - either a larger unit (6 - 8 kW), or a multi-split system with several indoor units.
However, this is only the starting point of the calculation. A real room always has a number of other factors that either increase this base figure, or, on the contrary, allow you to go slightly below it. We'll go through them one by one in the next section.
Diagram: guide output by room area
Factors that change the basic calculation
If it were enough to go by room area alone, choosing an air conditioner would be much simpler. In practice, however, a whole range of other circumstances affect the required output. Let's go through them one by one, with their real impact on the calculation.
1. The room's orientation to the compass points
A room facing south or southwest is exposed to significantly more solar radiation on summer afternoons than a room facing north. When choosing output, the following applies:
- north-facing room - the basic calculation is usually sufficient,
- south/southwest-facing room - add roughly 10 - 15 % on top of the basic output,
- a room with large south-facing glazed areas (e.g. French windows, a glass wall) - here an increase of even 20 - 25 % may be needed, because glass lets in far more heat than a solid wall.
Diagram: guide output increase by factor
2. Floor level and the roof above the room
A room under a flat roof or in an attic heats up noticeably faster and more in summer than a room in the middle of the building, surrounded on both sides by other heated/cooled spaces. If you're choosing an air conditioner for an attic room or a flat on the top floor with insufficient roof insulation, allow for an output increase of roughly 15 - 20 %.
3. Quality of insulation and windows
An older building with original (uninsulated) exterior walls and old, leaky windows loses heat faster in winter and, conversely, overheats faster in summer than a new building with insulation and quality plastic windows with insulated triple glazing. For a very poorly insulated room it's advisable to allow for higher output, whereas in a new building with good insulation you can sometimes get away with slightly undershooting the basic calculation.
4. Number of people and heat load from electronics
Every person in a room produces heat (roughly 80 - 100 W during sedentary activity, considerably more during physical activity). Ordinary electronics - TV, computer, games console, lighting - likewise increase the room's heat load. In practice this means:
- a typical living room for 2 - 4 people - the basic calculation is usually sufficient,
- an office with several computers, monitors and several people at once - add roughly 10 % to the basic output,
- a room with a server, multiple PC setups, or other technology with higher power draw - here you need to calculate the heat output of that equipment separately (as a rough guide, 1 kW of electrical power draw corresponds to roughly 1 kW of additional heat load).
5. Connection to other rooms (open-plan space)
If the living room is connected to the kitchen or dining room in a single open-plan space without separating doors, the output calculation must take into account the total area of this combined space, not just the part where the indoor unit will physically hang. This is one of the most common mistakes we see in practice - the customer calculates only "their" living room, but is actually also cooling the adjacent kitchen, which is freely connected through the open space.
Diagram: procedure for calculating the correct output
Practical examples from an everyday home
To make the selection process even more concrete, here are a few typical situations we encounter most often in the e-shop.
Example 1: 14 m² bedroom, north-facing
A typical bedroom in a flat, facing north, without large glazed areas, usually occupied by two people. The basic calculation (14 m² × 0.1 kW ≈ 1.4 - 1.7 kW) would theoretically be covered by an even smaller output, but in practice the smallest available unit class - i.e. around 2.5 kW - is usually recommended for such a room, since smaller units generally aren't manufactured, and the slight extra reserve also ensures quieter, calmer operation during sleep (the unit doesn't have to run at 100 % output even on the hottest nights).
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VAIB1-025WN outdoor and indoor unit with remote control - the smallest class in the Vaillant climaVAIR pro range, sized precisely for bedrooms and smaller rooms up to approx. 15 m², where lower output is sufficient but the customer still values quiet operation and quick control via remote. Price from EUR 527.00. |
Example 2: 22 m² living room, southwest-facing, large window
A typical living room in a new building with a larger window facing southwest. The basic calculation (22 m² × 0.1 - 0.12 kW ≈ 2.2 - 2.6 kW) would suggest a lower class, but due to the sunny orientation and larger glazing it's appropriate to add 15 - 20 % on top, which shifts the calculation to roughly 2.6 - 3.1 kW. In practice, a mid-range class of around 3.5 kW is therefore recommended for such a room.
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VAIB1-035WN outdoor and indoor unit with remote control - a mid-range output class that in practice handles a sunnier living room with larger glazing without you having to reach for a needlessly oversized unit. Price from EUR 561.00. |
Example 3: open-plan living room + kitchen, 38 m², attic
A larger, connected space in the attic part of the house, where heat lingers longer under the roof in summer than on lower floors. The basic calculation for 38 m² works out to roughly 3.8 - 4.6 kW; after adding 15 - 20 % for the attic location, we arrive at 4.4 - 5.5 kW. A more powerful unit is therefore appropriate for such a space.
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VAIB1-050WN outdoor and indoor unit with remote control - the most powerful in this range, well suited for larger connected spaces or rooms with a higher heat load (attic, sunny side, several people at once). Price from EUR 833.00. |
Comparing the three output classes
For a clearer picture, let's summarise the three units above in a simple overview by recommended room area and approximate price.
What happens when the air conditioner is undersized
An undersized air conditioner is, in practice, one of the most common causes of customer dissatisfaction. It typically shows up as follows:
- the unit runs practically non-stop at full output even during the hottest hours of the day, and the room still doesn't reach the set temperature,
- higher electricity consumption, because the compressor works at maximum for long periods instead of the more efficient medium load,
- faster wear of components due to constant load,
- on extremely hot days the unit "can't keep up" and the room temperature stays noticeably higher than you'd expect.
What happens when the air conditioner is oversized
A less well-known but equally real problem is the opposite extreme - a unit that's needlessly large for a small room. In practice this causes:
- short, frequent on/off cycles (so-called "short cycling") - the room cools very quickly, the compressor switches off, the temperature rises again, the compressor kicks in again - which is less energy-efficient and puts more strain on mechanical components,
- insufficient air dehumidification - an air conditioner dehumidifies the air precisely during a longer, smooth cooling cycle; with short cycles there simply isn't enough time for that, so the room can be cool but unpleasantly humid,
- a higher purchase price for output that will never actually be used in that room,
- in some cases also higher noise when the compressor starts up from idle (depends on the specific model).
So the golden rule is: the goal isn't to buy "as much output as possible just to be safe", but output that matches the room's real parameters as closely as possible, with a reasonable 10 - 20 % reserve for the factors mentioned above (orientation, floor level, glazing, number of people).
Specific situations worth considering
Rooms with a glass wall or patio doors
Large glazed areas significantly increase heat gain on sunny days. For such rooms we recommend not only increasing output according to the rule above, but also considering shading (blinds, films), which reduces the actual heat gain and makes the air conditioner's job easier without having to buy a needlessly oversized unit.
Offices and home offices with equipment
If you're planning an air conditioner for a home office where computers, monitors and other equipment run for long periods, factor in the additional heat load from this equipment (roughly the sum of the power draw of all devices in operation). For a single workstation with a monitor this is usually a negligible increase; for multiple PC setups or a server cabinet it's better to calculate the output separately, or consult a technician.
Several rooms at once - when to consider a multi-split system
If you need to cool several smaller rooms at the same time (for example two bedrooms and a home office), instead of several separate split units (each with its own outdoor unit for every room), it's often worth choosing a multi-split system in practice - one outdoor unit and several indoor units. The advantage is saving space on the facade and often lower operating costs; the disadvantage is a somewhat higher initial investment, and the limitation that if the outdoor unit fails, all connected rooms lose cooling at once. We cover this topic in more detail in a separate article on choosing an air conditioner for a flat or house.
The heating function and its effect on choosing output
If you plan to also use the air conditioner for supplementary heating during transitional seasons (spring/autumn), keep in mind that the heating output of an air-to-air heat pump (which heating-capable air conditioners rely on) gradually decreases at low outdoor temperatures. A model whose cooling output is exactly on the edge of what's needed may therefore have a real usable heating output lower than you'd expect on colder days. We cover this topic in detail in the article on air conditioners with a heating function.
Summary of the output selection process
- Measure the room's area (including any connected space, if it's not separated by a door).
- Calculate the basic output using the rule of roughly 1 - 1.2 kW per 10 m².
- Take into account the compass orientation and the size of glazed areas (10 - 25 % increase).
- Take into account the room's location (attic, top floor) and the quality of insulation.
- Take into account the number of people and the heat load from electronics, if significant.
- Choose a specific model with output as close as possible to the resulting figure, with a reasonable reserve, not a significant oversizing.
If, despite this process, you're still not sure which specific model would be best for your room, we're happy to advise you directly - just write to us via the form at the end of this article, or via the contact form on the product page, telling us what type and size of room it is.
Frequently asked questions
Is it enough to go by the room's area in m², or do I also need the volume (m³)?
At a standard ceiling height of up to 2.7 m, which is standard in most flats and family houses in Slovakia, it's perfectly fine to go by area in m². If you have a room with significantly higher ceilings (for example a loft, a studio, a space with a ceiling above 3 m), it's better to calculate using volume in m³ and increase the output accordingly, since the air conditioner has to cool a larger volume of air.
Is it better to choose a slightly higher output "just to be safe"?
A small reserve of 10 - 15 % above the exact calculation is fine and often even desirable. Significant oversizing (for example double the output actually needed), however, brings more downsides than benefits - frequent short cycles, worse air dehumidification, and a needlessly higher purchase price, as described above in the article.
Does the number of windows or the size of the glazing affect the output I need?
Yes, significantly. Large glazed areas, especially facing south or southwest, let considerably more heat into the room than a solid wall. For such rooms it's advisable to add 15 - 25 % to the basic calculation, as described in the section on room orientation above.
Can one air conditioner cool two rooms at once?
One indoor unit is designed to cool a single room (or an open connected space without doors). If you need to cool two separate rooms with doors that close, you need either a separate unit for each room, or a multi-split system with two indoor units connected to one outdoor unit.
How much does a poorly insulated building affect the required output?
For older, uninsulated buildings with original windows, it's worth allowing for a noticeable reserve above the basic calculation, since such spaces heat up faster in summer and the loss of cooling through walls and windows needs to be continuously offset. In practice we recommend consulting on the specific situation in such cases, since the exact increase depends on the condition of the particular building.
Can an air conditioner's output be "fine-tuned" after installation?
The unit's actual cooling output is determined by its design and cannot be increased or decreased after installation. What can be regulated is only what percentage of its maximum output the unit works at at any given moment (this is handled automatically by inverter control based on the current room temperature). That's why it's important to choose the right output at the time of purchase - no output adjustment is possible afterwards; the only remedy for a wrong choice is replacing the entire unit.
Related topics
- How to choose an air conditioner for a flat or house
- Wall-mounted vs. portable air conditioner
- Air conditioner installation - what you need to know
- Air conditioner with heating function
- Air conditioner operating costs
Back to the main category: Air conditioners and dehumidifiers
Have a question on this topic?
Not sure how to work out exactly what output you need for your particular room, or dealing with an unusual space (attic, open-plan area, several rooms at once)? Write to us - we're happy to help.