You’ve got two quotes in front of you for the same room — a 3.5kW and a 5kW — and the bigger one is only a few hundred dollars more up front. That gap is the smallest part of the difference. The running cost gap between a small and a large unit is in the hundreds of dollars annually, and over ten years that difference can approach the installation cost itself.
Split system running costs in Brisbane vary more by capacity than most homeowners expect. The size you choose is not just a comfort decision. It’s a recurring electricity bill decision that plays out every summer for the life of the unit.
What Each Size Band Costs
Consumer testing of split system running costs gives a clear picture of what different capacity bands cost per year. These are indicative ranges, not a quote for your specific home, but they are the right starting point for comparing options.
- Small — up to 4kW: roughly $150–$370 per year
- Medium — 4 to 6kW: roughly $340–$650 per year
- Large — over 6kW: roughly $530–$1,160 per year
Look at The Gap between the small and large bands. You’re not talking about a few dollars a year. You’re talking about hundreds.
Brisbane Runs Above the National Average
Brisbane sits in what consumer testing identifies as a “hot zone” for split system running costs. Our subtropical summers mean more hours of cooling demand than most of the country, so your real-world running costs will sit toward the upper end of those bands — not the midpoint.
When you compare two quotes, use the top of the range as your planning figure. A 3.5kW unit at the top of its band could run you $370 or more a year. A 6kW unit at the top of its band could run you $650 or more. Plan at the top, because that’s where Brisbane summers will put you.
Identical kW Can Mean Hundreds in Difference
Size is only half the decision. Two units with identical kW ratings can differ by hundreds of dollars a year in electricity, depending on their star rating and how their inverter behaves.
A high-star inverter unit running a variable-speed compressor will adjust its output to match the room’s load. A cheaper fixed-speed unit runs at full power until it hits the setpoint, then shuts off completely. That difference in operating behaviour shows up directly in your quarterly bill.
When comparing quotes, ask for the star rating of each model, not just the kW figure. Two units of the same size can cost you very different amounts to run.
Bigger Isn’t a Safety Margin
The most common mistake Brisbane homeowners make is treating a larger unit as a safety margin. “I’ll get the 5kW just to be sure” is a decision that will cost you money every summer.
A unit too big for its room short-cycles. It cools the space quickly, shuts off, then restarts a few minutes later when the temperature drifts. This constant stop-start cycle is less comfortable — the room fluctuates between too cold and too warm — and less efficient than a correctly sized unit running longer, steadier cycles.
Short-cycling also puts more wear on the compressor and its starting components. The unit that was supposed to give you headroom will likely need repairs sooner than a properly sized unit would.
Undersizing Costs More Than Its Band Suggests
The opposite mistake is equally costly. A unit too small for a Brisbane summer runs flat out for hours trying to reach the setpoint. It never gets there on the hottest afternoons, so you’re paying full power for a room that never fully cools.
An undersized unit costs more than its small size band suggests because it never cycles down. It runs continuously, pulling maximum current the whole time, and the compressor works harder than it was designed to. When you’re comparing two quotes, this is the trap to watch: the cheaper, smaller unit may look like the bargain until you realise it will run at maximum output for most of summer.
The answer is not to buy bigger. It’s to buy the size that matches the actual room load.
The Decision Rule: Calculate the Real Load
Your move before committing to either quote is to get the room’s real cooling load calculated. That means working through floor area, ceiling height, direct sun exposure, window size and orientation, and typical occupancy — not matching the old unit’s kW rating, and not copying what your neighbour installed.
These factors change what you need. A room with west-facing windows and a high ceiling needs a different unit than the same floor area in a shaded, standard-height room. The square footage alone doesn’t tell you what you need.
Once you have the calculated load, compare your two shortlisted sizes on purchase price plus ten years of running cost at the hot-zone figure. Write down both figures for each quote: the installed price, and the estimated annual running cost at the top of the relevant band. Multiply the annual cost by ten, add it to the purchase price, and compare the totals. That’s the number that matters — the purchase price and the ten-year running cost are one number, not two separate decisions.
How to Get the Sizing Calculation Done
The cooling load calculation is the first thing we do on a Brisbane split system installation. We work the room’s real load before recommending a capacity, so the unit we quote is the one that’s cheapest to own — not just cheapest to buy. The sizing is not a guess, and it’s not based on what you had before. It’s a measured exercise that gives you a defensible number to use when comparing quotes.
If you’re holding two quotes and aren’t sure which size is right, call 0420 718 827 and have the room sized properly before you commit. The conversation about your room’s dimensions, sun exposure and usage patterns will tell you which of those two units is the one you should be running for the next decade.
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