The most common air conditioning complaint we get called out to is not a broken unit. It is a unit that works exactly as designed and still does not keep the room comfortable, because it was the wrong size before it was ever switched on.
Sizing is the one decision in an air conditioning job that you cannot fix afterwards. The brand, the installer and the warranty are all replaceable. A unit that is two kilowatts short of the room is short forever.
Undersized and oversized both fail, differently
An undersized unit is the obvious problem. It runs flat out, never reaches the set temperature on the worst days, and wears itself out doing it. Running at maximum continuously is also the least efficient way to operate an inverter unit, so you pay more to be less comfortable.
Oversized is the mistake people do not expect, and it is the more common one because "a bit bigger to be safe" sounds sensible. A unit with far too much capacity cools the air to the set point very quickly, then throttles back or cycles. The thermostat is satisfied before the air has circulated properly and before much moisture has been pulled out, which in a Queensland summer is the half of the job that actually determines whether a room feels comfortable.
An oversized unit gives you a cold, clammy room with uneven temperatures, short cycling and higher running costs. The room reads 23 degrees on the wall and still feels wrong.
Start with the floor area, then adjust
As a starting point for a standard ceiling height, budget roughly 125 to 150 watts of cooling capacity per square metre of floor area. A 25 square metre bedroom lands somewhere around 3.1 to 3.7 kW; a 40 square metre open plan living area somewhere around 5 to 6 kW.
That is the first number, not the answer. The floor area tells you how much air there is. Everything that follows tells you how hard that air is to hold at temperature.
What pushes the number up
- West or north-west facing glass. Afternoon sun through a large window is the single biggest load in most coastal homes and can add a kilowatt or more on its own
- Large glazed area generally, especially single glazed, and especially unshaded
- High, raked or cathedral ceilings, which add volume the floor area does not capture
- A poorly insulated or uninsulated roof space, which radiates heat down all afternoon and well into the evening
- Open plan, where the room does not stop at a door and the unit is really conditioning a larger space
- Kitchens, where the cooking, the oven and the fridge all add heat
- A room that is regularly full of people, like a home office with two desks or a media room
What pulls it down
- South facing rooms with little direct sun
- Good ceiling insulation, eaves, awnings, external blinds or established shade
- A single, well sealed room with the door normally closed
- Ground floor rooms in a two storey house
Humidity, which the floor area ignores entirely
On the Sunshine Coast, latent load — moisture — is a real part of the calculation. A unit sized tightly to the sensible heat load will run longer, cycle less and dehumidify properly. This is precisely why rounding up "to be safe" backfires here.
Do the measurement, not the guess
A proper sizing is a short calculation, not a vibe. We measure the room, note the ceiling height and construction, record the orientation and the glazed area, check whether the roof space is insulated, ask how the room is used and how many people are normally in it, and size from that.
It takes about fifteen minutes on site and it is the difference between a unit that quietly does its job for fifteen years and one you are annoyed with every February. Anyone quoting a unit size over the phone from a floor area alone is guessing, and the guess is usually rounded up because a bigger unit is a bigger invoice.
The electrical side, which decides whether it can be installed at all
A split system needs its own dedicated circuit with correct protection and isolation. Two things follow from that.
First, your switchboard needs a spare way and the capacity to carry the load. A board that is already full, or still running ceramic fuses, has to be addressed before the aircon goes in — which is why that work sometimes appears in an air conditioning quote and sometimes, on a cheaper quote, quietly does not.
Second, the bigger the unit, the bigger the load. This is another argument against oversizing: a unit two sizes up may push you from a straightforward install into a switchboard upgrade. Our walkthrough of what happens during a switchboard upgrade covers that work.
Cheaper ways to make the number smaller
Before you size up, it is worth reducing the load instead. It is almost always the better spend.
- Ceiling insulation. The highest return item in most older coastal homes
- Shade the west facing glass — external blinds, awnings or planting beat anything you can do from inside
- A ceiling fan alongside the split system. Moving air lets you run two or three degrees warmer for the same comfort, which takes real load off the unit through a long summer
- Seal the obvious gaps around doors, windows and exhaust fans
Do these and a correctly sized unit will outperform an oversized one in an unimproved room, for less money and less running cost.
One unit or several?
A single large unit cannot push conditioned air around corners. If you are trying to cool an open plan living area plus two bedrooms down a hallway, one big head unit in the living room will leave the bedrooms warm no matter what capacity you buy. Multiple smaller units, a multi-head system or ducted are the honest answers, and which one suits depends on the layout and the budget. We will tell you which, including when ducted is not worth the premium for your house.
Getting it quoted properly
We size the unit on site, quote the unit and the electrical work together, and tell you up front if the switchboard needs attention so there is nothing to add later. See our air conditioning services for installation and servicing, and read what air conditioning installation costs for what drives the price and what gets left out of cheap quotes.
If the running cost is what is really bothering you, cooling is the load that pairs best with generation — our notes on whether solar is worth it explain why running your aircon on your own power changes the maths.
Want it sized properly, on site?
We measure the room, check the orientation, glazing and roof insulation, then quote the unit and the electrical work together. Fifteen minutes now saves you fifteen summers of annoyance.
Written by Scott Gross, licensed electrician and founder of PMU Electrical, servicing the Sunshine Coast and Brisbane. Licence 146529.