You walk into an appliance store to buy an air conditioner. The salesman asks you the classic, expected question: "How many square metres is your room?". You reply "About 20 square metres", and he immediately points you towards a 9,000 BTU unit.
It sounds logical and fast. However, this approach is the number one reason thousands of consumers end up with air conditioners that either "freeze" the room clumsily, or run non-stop at full blast consuming enormous amounts of electricity, without ever adequately cooling the space.
As engineers, we must clarify the following: Square metres are only the starting point of the calculation, not the end. Let us look at what BTU really means, why the "rule of thumb" often fails, and what are the 4 hidden factors you must consider before making your purchase.
The abbreviation stands for British Thermal Unit. It is the unit of energy measurement used worldwide in air conditioning. In practical terms, it shows us the "capacity" (power) an air conditioner has to remove heat from your room within one hour.
The BTU number indicates how much thermal energy the air conditioner can transfer from indoors to outdoors within one hour. The typical residential categories are 9,000, 12,000, 18,000 and 24,000 BTU. Each category covers a specific range of spaces, but the final choice depends on much more than simple square metres.
To get an absolutely basic order of magnitude, engineers use a quick multiplier: we roughly calculate 250 to 300 BTU per 1 square metre for a room with a standard ceiling height (around 2.80 metres).
15 × 300 = 4,500 BTU. We go up to the smallest available, i.e. 9,000 BTU. A typical bedroom in a mid-range apartment, with no particular thermal loads, is comfortably covered.
25 × 300 = 7,500 BTU. We go for 9,000 BTU or marginally 12,000 BTU. It depends on the orientation and insulation as to whether an "upgrade" is needed.
40 × 300 = 12,000 BTU. We go for 12,000 BTU or 18,000 BTU. Here, large windows, the kitchen and the number of occupants begin to play a significant role.
60 × 300 = 18,000 BTU. We go for 18,000 BTU or 24,000 BTU. In such a large space, a proper engineering study is virtually mandatory.
💡 This rule works excellently... only if your home has perfect insulation, is never hit by sunlight, and nobody cooks inside it! Since we live in the real world, this is where thermal loads come into play.
To find the real air conditioner size you need, you must ask yourself the following four questions. The answer to each can dramatically change the result.
A 15 m² bedroom facing north that is permanently shaded is comfortably covered by a 9,000 BTU unit. The very same room (15 m²), if facing south-west, with huge glazing and hammered by the midday and afternoon sun in August, needs almost 30% more cooling capacity. There, a 9,000 BTU unit will struggle, and you probably need 12,000 BTU.
If you live in an old building (pre-1980) with single glazing, draughty wooden windows and uninsulated walls, the coolness from the air conditioner "escapes" instantly. Conversely, in a new building with external insulation and energy-efficient windows, losses are minimal. Tip: If your space is the top floor (penthouse) and the roof above has no insulation, add at least 15% to the required BTU. The roof in summer acts as a huge radiator emitting heat downwards.
Which room will the unit go in? If it is for the open-plan living-kitchen area, you must account for the heat generated by appliances. An oven that is baking, hobs that are boiling, even the fridge or a large television, produce enormous amounts of heat. A space with a kitchen always requires "one size up" air conditioner compared to a plain living room.
The human body, even when sitting relaxed on the sofa, emits heat (approximately 400 BTU per hour, per person). If your living room regularly hosts a family of five, or you frequently entertain guests, the air conditioner must have the power to absorb this extra heat (latent load).
Choosing the wrong air conditioner size has two faces, and both are equally destructive. Let us look in detail at what happens in each case.
You put a 9,000 BTU unit in a 35 m² living room because it was on sale. The machine will never reach the desired temperature (e.g. 26°C). The compressor will run non-stop at 100% of its capacity, electricity consumption will skyrocket, and the machine will break down much sooner due to wear.
The mentality of "let me get the 24,000 BTU to be safe, even though the living room is 25 m²". This is perhaps the biggest mistake in air conditioning. An oversized unit will drop the air temperature abruptly in just 5 minutes and shut off. However, it won't have time to dehumidify. The room will be freezing cold but with a "heavy", damp atmosphere. Furthermore, the abrupt on-off cycling destroys the core advantage of Inverter technology.
To choose the right air conditioner, look at your windows, think about when the sun hits, and whether your roof is insulated. If you are borderline between two sizes (e.g. between 12,000 and 18,000 BTU), in an uninsulated, sun-drenched space go for the larger one. In a shaded, well-insulated space, stick with the smaller one.
Now that you know how many BTU you need, it is time to look at the label with the unit's energy characteristics. And there hides yet another alphabet: SEER, SCOP, A+++. In our next article, we decode exactly what these abbreviations mean and how to avoid being misled by the energy label.
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