2.5Kw for a bedroom, 5Kw for a garage sized room and a 7Kw for a big lounge. Sounds simple enough and in some cases that might work however there are other factors to consider when trying to find the correct air conditioner unit size for a room.
The size and shape of the room play the major part, however room insulation and the amount of sunlight also play an important role. The orientation of the room also makes a huge difference.
Installing an Air Conditioning Unit that is too big for a room seems like a good idea as it will cool the room down quicker, however, there may be side effects. If the unit is too over sized, it will cool the room to fast not allowing the moisture to be removed from the room. The room will feel cool however the room will feel uncomfortable due to the humidity being too high.
Buying an Air Conditioning Unit that is too big will also cost you more to buy the unit. The installation price may be higher also. The unit may also not have as good an energy star rating costing more to run.
Putting in an air conditioning unit that is to small has the obvious disadvantage of working too hard to achieve the desired temperature. A unit that works to hard will have a lot more wear and tear. Running costs will be higher and you may void the warranty due to it being undersized and being overworked.
A correctly sized air conditioning unit will cool the room in a reasonable enough time to remove the humidity. Once the room is at the correct temperature the compressor will wind down allowing it to maintain the room temperature and allowing for maximum energy savings. This is where the inverter compressor really shines – as its energy saving are best at this reduced load.
TYPE OF ROOM: What is the room being used for? Bedrooms have a smaller heat load as they are typically used only at night. Rooms with a number of heat sources (people or computers) may need to be factored into the calculation.
ORIENTATION OF ROOM: Rooms that face east usually receive the morning sun and west facing rooms receive more sun in the afternoons. North facing rooms receive sun most of the day. South facing rooms usually need less cooling as they do not receive any direct sun.
GLASS WINDOWS: Glass windows allow a lot of heat transfer from outside. Ideally double glazing will be optimal as it will drastically reduce this heat transfer. However this is not usually viable due to the cost. This is where internal and external shading becomes necessary.
SHADING: External shading such as blinds, awnings and shadows can drastically reduce the amount of heat build-up in a room. Internal shading such as curtains and blinds also make a difference. The more direct sun you can keep from entering your house the cheaper it will be to keep it cool or warm.
INSULATION: Rather than relying on a thin layer of plasterboard between the hot roof space air and the inside of your house, having a layer of ceiling insulation will make a huge difference. There can be a 10 degree difference between the interior of a house with and without ceiling insulation. Insulated walls can also make a huge difference. Apartment usually have concrete floors and ceiling and this is also a good insulator.
SEALING: Large open rooms will not hold the heat as well as a bedroom with a closed door. Most open plan living areas open up to hallways and other rooms without doors. Gaps under doors and between rooms can all add up to heat transferring between rooms.
INDOOR POSITIONING: Although not so critical in bedrooms as they are usually small square shaped rooms the positioning of the air conditioning unit in the larger rooms can help a lot. The most central location can help even air flow which helps with an even temperature in the room.
OUTDOOR POSITIONING: Most outdoors positions are suitable for residential air conditioning units. That is because they are usually placed against a wall. If you are putting the outdoor unit on a roof facing it blowing north is usually best. This is because you do not want any direct sun blowing on the back of the coil. The coils transfer heat and if the sun is shining on these coils it can add an extra load on the system having to get rid of this heat.
EFFICIENCY: Almost all air conditioning units are inverter models these days. These have a compressor which regulates its speed as to how much heating or cooling it needs to do. These variable speed compressors have a huge energy savings compared to the older either ‘on or off’ models.
Energy ratings are usually similar throughout the size ranges. With the smaller 2.5Kw units usually having about 5 stars for cooling and heating. By the time you get up to 7kw models the star ratings drop to about 2.5 stars. And usually about 2 for the 8 and 9Kw models.
Weather its a cheaper Kelvinator or more expensive Daikin unit they usually have similar energy ratings.
The air conditioner unit size needs to be calculated for the room.
We need to find the area of the room. This is calculated by length x width.
Ceilings are usually about 2.4 meters high. Anything higher than this should be added to the calculation. Cathedral ceiling usually have more heat transfer due to having no ceiling space and being closer to the roof tiles or tin above.
After finding the area of the room we need to multiply this by the amount of watts per square meter. The number of watts depends on the room.
A bedroom can be calculated at about 0.13 Kw to cool 1 x square meter – this has a lower value as most bedrooms are only used at night times when there is not a large heat load on the building.
Living areas can be calculated at about 0.15 Kw to cool 1 x square meter.
The Kw per square meter may be more depending on room insulation, sunlight and room orientation. The height of the ceiling can also affect the calculation.
1: Measure the width of the room in meters the air conditioner will be in.
2: Measure the length of the room in meters the air conditioner will be in.
3: Multiply the length and the width of the room to find the area of the room.
4: Multiply the area of the room by the Kw per square meter 0.13 – 0.15Kw
Example: You have garage that you want to air condition.
1: Measure the width of the room -The garage is 6 metres wide.
2: Measure the length of the room -The garage is 6 metres long.
3: Multiply the length and the width of the room.
The garage is 6 metres long and 6 metres wide.
6 x 6 = 36
The area that you need to cool would be 36 metres squared.
4: Multiply the area of the room by the Kw per square meter.
If you multiply 36 by 0.15 you will get 5.4.
You would need an air conditioner size unit with a cooling capacity of about 5.4kW.
You may be able to use a 5Kw unit if the room is fairly insulated and has no direct sun.
If you have a large panel garage door that receives the direct sun you would be better off choosing a air conditioner size unit of about 6Kw.
| Room size | Capacity |
| Small Up to 20m2 Bedroom, study, small kitchen | 2–2.5kW |
| Medium 20–40 m Bedroom with ensuite, small lounge | 2.5–5kW |
| Large 40–60 s m2 Large bedroom, mid-sized lounge, large kitchen | 5–8kW |
| Extra Large 60–80 m2 Open-plan areas, large lounges | 7–9kW |