What size air conditioning unit do I need?

Woman Looking at Air Conditioners

Working out what size and power level of air conditioning you need is surprisingly straightforward. As recommended by Which? in its recent review of best air conditioners, a good rule of thumb is simply to multiply by five the dimensions (in feet) of the room you want air conditioning. This will give you the BTUs (British Thermal Units) you require.

For example, a room measuring 15 x 10 x 8ft: (15 x 10 x 8 x 5) requires an air conditioner of 6,000 BTUs.

What’s a BTU?

BTU stands for British Thermal Units – this measurement refers to thermal heat energy and in turn the power of an air conditioning unit and in turn how much energy it uses.

It is important that you specify the correct BTU because an air conditioner that is too powerful will cool your home too quickly. This is a problem as your air conditioner also removes moisture as part of the cooling process and if it shuts off too quickly it won’t have time to remove the moisture from the air. This in turn will keep your air conditioning unit powering on and off as it tries to maintain the optimal temperature. Accordingly, it will wear out far more quickly.

On the other hand, too few BTUs will mean not enough power. This will force your air conditioning to run constantly in an effort to maintain the required temperature. This too will cause the air conditioner to wear out much faster because of the increased use – and worse still, it will drive up the costs of your energy bills. 

Energy efficiency

Energy efficiency is key to keeping your bills low while helping you do your bit for the planet. Making sure your air conditioner has the right cooling capacity in BTUs is the first step; next, you should look for its energy efficiency ratio (EER) rating.

So, how do you establish the energy efficiency ratio of an air conditioning unit? Basically, it is the ratio between its BTU and its power input (in watts). Generally, the higher the EER rating, the more efficient the air conditioner.

You can easily spot the EER rating of different air conditioning units by the rating label they carry. Here at MAC we only install systems rated A+ or above, which is the highest energy saving classification.

Here is a guide to the EER ratings of the makes and models of air conditioners we recommend:



EER Ratings

Mitsubishi Electric


A+ to A++


A+ to A+++


A++ to A+++



A+ to A++



A+ to A++

For more information, please see links to supplier product pages/spec sheets below:

Mitsubishi Classic

Mitsubishi AP

Mitsubishi LN

Daikin Sensira

Toshiba Seiya

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