Having air conditioning installed is a worthwhile investment, a significant expense, and a decision that requires careful planning.

You may already have done some research yourself, perhaps you’ve looked at how air conditioners work, and which size and types you favour. There may be some features that you consider are important, such as running costs, aesthetics, or smart technology.

However, bad choices can lead to disappointing results, and soaring energy bills, something we that all wish to avoid. In this article we explain why buying an air conditioner with the correct output, should be top of your list, and a crucial part of the selection process.

Does size matter?

Yes, we know it’s a cliché, but size matters. In fact, it is the most important consideration.

An air conditioner that’s too small will not heat or cool your room effectively, and one that’s oversized will be inefficient and expensive to run. It important that we first establish that size in terms of physical dimensions, and size in terms of output, are two separate entities.

From now on we want you to think of size as your unit’s ability to produce cool and/or warm air. In this industry we measure size in kW (kilowatts), and sometimes BTU (British Thermal Units).

Simply put, size is all about output, or in technical terms, the duty. The duty has a direct impact on how your AC system will perform in terms of efficiency and running costs.

Bigger is not necessarily better

A unit that is oversized for an area can put too much pressure on the system’s components. It will be costly to run, and costly to repair, if your system packs up.

Physical size doesn’t indicate the efficiency or power of a unit, technology advancement means that some of the smaller more compact designer air conditioners can offer much more than the older bulkier systems.

An oversized system will stop and start more often. This is not energy efficient and will lead to increased energy bills. Not only that, but you will also struggle to maintain an optimum temperature and will often find that the room is too hot or too cold.

The correct size ac unit will effortlessly keep your room at the optimum temperature.

Undersized unit = oversized bills

An undersized AC unit will need to run constantly, and will not be powerful enough to reach, and maintain, the optimum room temperature. This will mean higher energy bills. An undersized unit that is having to work too hard, can also hit your wallet hard in terms of maintenance costs, and could result in your unit having premature wear and tear, breakdowns and a shortened life span.

Other factors that will affect size …

There are several variables to consider that will impact selection of the duty (kw or BTU) of an air conditioner:

How well insulated is the building?

Adequate insulation directly affects the efficiency of your air conditioner, if you do not have a well-insulated property this will need to be considered when calculating kw size. Poorly insulated homes or workplaces may require an AC system with a higher output.

Is the room south-facing?

South facing rooms are exposed to more sunlight and heat, which means that you will have more heat gain. Your AC system will need to work harder to keep south facing rooms cool and you would need to make additional allowances in your calculations for this.

How much of the external walls are made of glass?

The size and orientation of the windows is especially important. South facing windows result in high solar gain which will be taken into consideration when calculating KW/ BTU size. You may need a more powerful unit.

Where is the room located?

Warm air rises, so an air conditioner in a loft space will need more cooling capacity than a ground floor room.

Is there electrical equipment within room?

Rooms such as comms rooms or entertainment rooms contain heat generating electrical equipment, and this increased need for cooling will need to be catered for when choosing the size of your air con unit or units.

Is the room an above average height?

A room with a high ceiling has much more volume than a room with a standard ceiling height and a larger unit may be considered more suitable here.

How to calculate the correct size …

Use our simple calculator to estimate the correct size air conditioner for your room

This calculator uses the simple formula of 125 watts of cooling per square metre to estimate the best size for your space, however it is important to remember that there are other variables (such as those above) that will need to be factored in.

Always round up to the nearest 0.5kw

This is for estimation purposes only; a qualified engineer will be able to tell you the exact size for your individual requirements.

Here is an example of how to estimate AC size for a room …..

A room that meets standard criteria of 5 metres in width 7 metres in length which gives me a total area size of 35m2. Based on 125 watts per square metre I would need a minimum of 4.375kw of cooling.

Simply …
5 x 7 = 35 x 125 = 4,375 (watts)

Air conditioning unit size required = 4.5kw or around 15,300 BTU. Click below to see BTU to KW conversion chart.

BTU to KW Conversion chart

BTU	KW
6000	1.76
8000	2.34
10000	2.93
12000	3.52
14000	4.10
16000	4.69
17000	4.98
18000	5.28
19000	5.57
20000	5.86
24000	7.03
27000	7.92
30000	8.79
32000	9.39
36000	10.55

How much does air conditioning cost?

Commercial installations

Each commercial air con installation will present with entirely different heating and cooling requirements.

To meet these diverse applications there are a range of air conditioners available with different set ups and layouts, from single split systems to complex VRF or VRV systems.

As one size does not fit all, it would be difficult for you to estimate the cost of your installation. Therefore, it is essential that you enlist the expertise of an experienced and accredited AC installer, who will confidently identify your needs and design a tailor-made air con system to fit your individual circumstances perfectly.

Domestic installations

Firstly, use our size calculator to estimate the size unit you will require. Then refer to the table below to get an idea of your air conditioning cost of installation.

Remember when considering size, it is important to consider the different variables listed above.

This table is for estimation purposes only, the final price of your install will be dependent on which manufacturer you choose the pipe run (distance from indoor to outdoor) and which type of unit you opt for (e.g. wall mounted, ducted).

ESTIMATED INSTALLATION COST FOR A HOUSE

UNIT SIZE KW	BUDGET COST
2	£1200 - £1600
2.5	£1300 - £1700
3.2	£1400 - £1800
4	£1500 - £1900
4.5	£1600 - £2000
5	£1700 - £2100
6	£1800 - £2200
7	£1600 - £2000
8	£1700 - £2100
10	£1800 - £2200

If you would like any further advise on which size and type of AC system is perfect for you, please contact our specialist team at Evergreen Air Conditioning Limited. We offer a free survey carried out by an experienced engineer followed by a free, comprehensive, no obligation quotation.

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