The cheapest quote on the table is often the one for a system a size too small. It will work most of the year, which is exactly what makes the problem so hard to spot until the weather turns.
Sizing is the least glamorous part of a commercial air conditioning project and the one most likely to cost you for the next fifteen years. Get it right and the system holds temperature without straining, runs efficiently and lasts. Get it wrong and you are paying for the mistake on every power bill, in every service call, and on the handful of days each year when the building most needs to perform.
So when a business owner asks what size air conditioner do I need, the honest answer is that nobody can tell you from the floor area alone. Here is where the number comes from, and what happens when it is wrong in either direction.
Quick summary
- Commercial air conditioning sizing comes from a calculated heat load, measured in kilowatts, not from a rule of thumb applied to floor area.
- The load depends on floor area and ceiling height, glazing and which way it faces, insulation, how many people use the space, heat from equipment, and the local climate.
- An undersized air conditioner runs flat out, costs more to run and gives up on the hottest and coldest days of the year.
- Oversizing causes its own problems: the system short cycles, wears faster and controls humidity poorly, which matters a great deal on the Mid North Coast.
- Eastland carries out load calculations as part of quoting commercial air conditioning across Coffs Harbour and the Mid North Coast.
How is commercial air conditioner size worked out?
It is worked out from a heat load calculation, which adds up everything that puts heat into the space and everything that lets heat escape, then sizes the equipment in kilowatts to handle the result at the worst conditions the building will realistically face.
Why the per-square-metre rule of thumb falls short
Rules of thumb exist because they are quick, and they are roughly serviceable for a plain residential bedroom. Commercial spaces break them almost immediately.
Two rooms of identical floor area can have wildly different loads. One has a three-metre ceiling, full-height west-facing glass and thirty people in it. The other has a standard ceiling, no external windows and four people. The floor plan says they are the same. The equipment they need is not remotely the same. Anyone quoting from area alone is guessing, and the guess is usually rounded in whichever direction wins the job.
What goes into a thermal load calculation
A proper thermal load calculation accounts for:
- Floor area and ceiling height, which give the actual volume of air to condition.
- Glazing: how much, which orientation, and whether it is shaded. West-facing glass in a Coffs summer is a heat source in its own right.
- Building fabric and insulation, including roof construction and wall type.
- Occupancy, because every person in the space is a continuous heat source.
- Equipment load: commercial kitchens, server rooms, display lighting and machinery can dwarf everything else on the list.
- Fresh air and ventilation requirements for the building’s use.
- Local climate data, for both the cooling peak and the winter heating load.
That last point gets forgotten. Reverse-cycle equipment has to carry the heating load through winter as well, and a system sized only for the summer peak can be the wrong answer in July.
The weighting shifts with how a space is used. Working out the right air conditioner size for office areas usually turns on occupancy and equipment heat, since people and hardware dominate the load. In retail it is more often glazing, door traffic and lighting. In hospitality the kitchen changes the picture entirely.
What an undersized system costs you
Undersizing is the more common fault, because it is the one that makes a quote cheaper.
The trap of the slightly cheaper quote
Dropping a system down one size can take a few hundred dollars off an installed price, and it is close to invisible on a quote unless you know to look. The result is a system that copes perfectly well for most of the year, which is precisely why the problem goes unnoticed until a heatwave, a full room or a cold snap exposes it.
A system that manages eighty or ninety per cent of the time sounds acceptable until you work out that the remaining ten per cent lands on your busiest trading days and your most uncomfortable mornings.
It never gets to rest
An undersized unit runs continuously trying to reach a set point it cannot achieve. Compressors and fans that never cycle off wear out faster, which brings forward both repairs and replacement. Running flat out for hours also consumes far more power than a correctly sized system cycling comfortably, so the saving on the install quietly reverses itself.
Oversizing causes its own problems
Going too big is not the safe option people assume it is, and on a humid coastal site it can be the worse mistake of the two.
Short cycling
An oversized system hits the set temperature quickly, shuts down, then restarts a few minutes later. Those frequent starts are the hardest thing a compressor does, so wear accumulates faster than on a system running longer, steadier cycles. You also get noticeable temperature swings rather than stable comfort.
Poor humidity control on a subtropical coast
This is the one that catches people out locally. Air conditioning removes moisture from the air while it runs, and that takes sustained run time. A system that blasts to temperature and switches off has not run long enough to pull the humidity down, so the space hits the right number on the thermostat and still feels clammy and unpleasant.
On the Mid North Coast, where summer humidity is a genuine feature of the climate, an oversized system can leave a building that is technically cool and practically uncomfortable.
| What you are noticing | What it often points to |
|---|---|
| Runs constantly on hot days and never reaches the set point | Undersized for the load |
| Cools quickly then switches on and off repeatedly | Oversized for the load |
| Right temperature but the air feels damp and heavy | Oversized, or poor run time for dehumidification |
| Fine in most rooms, poor in one area | Zoning or distribution issue rather than capacity |
| Was adequate for years, struggling now | Changed use or occupancy, or a system losing performance |
If any of those sound like your building, it is worth having the load checked against what is actually installed. Talk to our team and we can work out whether you have a sizing problem or a service problem.
Why Mid North Coast sizing is not the same as Sydney sizing
Generic calculators and interstate rules of thumb consistently under-read the load on this stretch of coast, for a few reasons that compound.
Humidity is the big one. Removing moisture is real work, and a coastal subtropical summer asks a great deal more of a system than a dry inland climate at the same air temperature. Then there is the coastal exposure itself: salt air is hard on outdoor coils and condensers, so a system already running at its limit degrades faster here than it would further inland.
Building stock matters too. Plenty of commercial premises around Coffs Harbour were built or fitted out before current insulation and glazing standards, which changes the fabric load considerably. Sizing off a modern assumption in an older building is a reliable way to end up short. For larger sites, that calculation feeds directly into how industrial air conditioning gets specified as well.
How Eastland approaches commercial air conditioning sizing
Sizing is part of quoting, not an optional extra or something worked out on site on installation day.
The load calculation happens before the quote
We carry out thermal load calculations as part of preparing a quote for commercial air conditioning, so the capacity we propose is derived from your building rather than from the last job that looked similar. Alex does the majority of quoting himself, which means the person assessing the space is the person accountable for whether the system performs.
We will not drop a size to win on price
This is where we lose some jobs, and we have made our peace with it. If a competing quote is cheaper because it specifies a smaller system, the comparison is not like for like, and we would rather explain that than match it. A system that fails on the days you need it is not a saving, and we are not interested in being the company that installed it.
Sizing on new builds and fit-outs
On design-and-construct and tender work with builders and architects, we do the load calculations as part of the design so the mechanical services suit the finished building rather than the drawing. Once a system is commissioned, a maintenance plan keeps it performing at the capacity it was specified for, since a neglected system effectively becomes undersized over time.
Frequently Asked Questions
How is air conditioning capacity measured?
In Australia, capacity is quoted in kilowatts, and reverse-cycle systems carry separate cooling and heating figures because the two are rarely identical. You may still see BTU per hour on imported equipment or older documentation, which is the imperial equivalent. What matters when comparing quotes is that you are comparing the same measure, and that the kilowatt figure quoted is the capacity at realistic local conditions rather than at a laboratory rating.
Can I just replace my old unit with one the same size?
Often, but not automatically, and it is worth a check rather than an assumption. The original system may itself have been sized poorly. More commonly, the building has changed since: a fit-out altered the layout, occupancy went up, new equipment added heat load, or glazing and insulation were upgraded. Like-for-like replacement carries forward whatever was wrong with the original decision, so a fresh calculation on a system you are keeping for the next decade is cheap insurance.
What information does an installer need to size a system properly?
Useful things to have ready: floor plans with dimensions and ceiling heights, the orientation of the building, where the windows are and whether they are shaded, how many people occupy the space at peak, what equipment runs in there, your operating hours, and any known problem areas. None of it replaces a site visit, but it makes the assessment faster and the quote more accurate. If an installer asks for none of this, that tells you something about how the number was arrived at.
Does the correct air conditioner size actually reduce running costs?
Yes, on two fronts. A right-sized system spends more of its time cycling at efficient part load rather than running flat out or stopping and starting. It also lets you hold a sensible set point, which is where the real savings sit: Australian Government guidance notes you can save up to 10 per cent on cooling for each degree you raise the thermostat. A system that is struggling cannot give you that choice, because staff will push the set point further just to get a result.
On a new fit-out, who is responsible for getting the sizing right?
It depends on how the contract is structured, and it is worth clarifying early because it is a common gap. On a design-and-construct job, the mechanical contractor generally carries the design responsibility, including the load calculation. Where a builder is working to a consultant’s mechanical drawings, the sizing decision was made upstream and the installer is delivering to that specification. Problems tend to arise when nobody is clearly holding it, so establish at tender stage who is doing the calculation and who stands behind the result.
Get the sizing right before you commit to a system
A correct air conditioner size is not a technical nicety. It is the single decision that determines whether the system is comfortable, affordable to run and long-lived, or a fifteen-year irritation you paid a little less for.
For a properly calculated quote on your building, call Eastland on 0488 549 288 or request a quote through our contact page, and we will assess the space before proposing anything.
We design, install, service and repair air conditioning in Coffs Harbour and across the Mid North Coast, for offices, retail, hospitality, government and industrial sites, alongside our commercial refrigeration work. Fully licensed, ARCtick certified and insured, with after-hours emergency breakdown cover when you need it. And when you deal with Eastland, you deal with the owner, not a call centre.
You can also read what other Coffs Harbour and Mid North Coast businesses say about us on our Google Business Profile.
This article is general information only and does not account for the specific conditions of your site. For a capacity recommendation specific to your building, book an on-site assessment with Eastland.
References
- energy.gov.au: Heating and cooling, Australian Government guidance on choosing the right sized appliance and the running cost effect of each degree on the thermostat.
- Australian Refrigeration Council (ARC), the national ARCtick licensing scheme, including the online licence check for verifying an installer.