What Size Air Conditioner Does Your Home Actually Need?
Airtight, well-insulated homes need far less heating and cooling than conventional sizing rules suggest. Estimate your load and see a suggested split system size range.
“What size air conditioner do I need?” is usually answered with a rule of thumb — so many kilowatts per square metre of floor area. That rule was written for a typical Australian home: moderate insulation, leaky construction, standard glazing.
An airtight, well-insulated home is a different building. It loses far less heat in winter and gains far less unwanted heat in summer, so the same floor-area rule of thumb usually overstates what it actually needs — sometimes by several times over.
This calculator gives a rough, planning-stage estimate of your home’s heating and cooling load based on its envelope, glazing, airtightness and location — so you can start the conversation with an installer from a more realistic number.
Estimate your heating & cooling load
The defaults represent a typical 180 m² Adelaide home with double glazing and a reasonable airtightness result. Adjust the sliders and dropdowns to match your own home or plans.
Heating & Cooling Size Estimator
Enter your home’s envelope and location to see a rough heating and cooling load estimate and a recommended system capacity.
House & location
Mild winters, hot dry summers
Building envelope
Ventilation & comfort target
Estimated heating load
3.6–4.8 kW
Estimated cooling load
6–8.1 kW
Shown as a range because glazing performance, airtightness behaviour and solar gains carry real uncertainty ahead of a detailed room-by-room design — treat the range, not a single number, as the honest answer.
Where your cooling load comes from
Roof and floor make up 51% of your cooling load. Improving these areas could reduce the required cooling capacity.
🟠 Extra heat adding to your cooling load: 2.8 kW
- • Occupants — 0.30 kW
- • Appliances & standby — 0.15 kW
- • Summer sun — 2.33 kW
What if you improved...
Recommended system capacity
Around 7.5 kW
Manufacturer ratings vary — closest common product steps are typically 7.1–8.0 kW.
A conventional installer working from floor-area rules of thumb may suggest something closer to 13.4 kW — high-performance homes often need smaller, better-controlled equipment than that.
Ready for an accurate heating and cooling design?
We calculate heating and cooling loads from your plans, insulation, glazing, orientation and airtightness target — not just floor area.
Educational estimate using a simplified single-zone model — not a replacement for a room-by-room heating and cooling load calculation. Actual requirements depend on your specific plans, glazing schedule, measured airtightness and local microclimate.
Why the numbers come out smaller
A building’s heating and cooling load is driven by how much heat moves through its envelope, how much air leaks in and out, and how much unwanted (or welcome) solar heat comes through the glazing. Four factors compound to shrink that number in a high-performance home:
Airtightness
Less uncontrolled infiltration means less conditioned air escaping — and less unconditioned air replacing it. This is measured directly by a blower door test and expressed as ACH50.
Insulation
A better-insulated envelope slows heat transfer through walls, roof and floor, reducing the temperature difference the mechanical system has to fight.
Glazing
Double or triple glazing loses far less heat than single glazing, and the right glass and shading combination controls unwanted summer solar gain without blocking useful winter sun.
Orientation & shading
Well-designed eaves and glazing placement admit low-angle winter sun for free heating while blocking high-angle summer sun that would otherwise become a cooling load.
Why this matters at quoting stage: if an installer sizes your system off a standard floor-area rule of thumb without accounting for airtightness, insulation and glazing, you can end up paying for — and living with — a system several times larger than the home needs. See our full guide on choosing the right heating and cooling system for a high-performance home for the complete picture on split systems, bulkheads, ducted and hydronic options.
What actually goes wrong with an oversized system
Short cycling: An oversized unit reaches its target temperature quickly, then switches off — then on again soon after. That start-stop cycling is harder on the equipment and less efficient than a longer, steady run.
Poor dehumidification: Removing moisture from the air takes time and steady airflow. A system that satisfies the thermostat in minutes doesn't run long enough to properly dehumidify, leaving the air feeling clammy even at the right temperature.
Noise and cost: Bigger compressors and fans move more air and refrigerant than the room needs, and cost more to buy — for a smaller job than the size suggests.
Lower part-load efficiency: Most reverse-cycle systems are least efficient running at partial load, which is exactly where an oversized unit spends nearly all its time.
Related reading
Heating & Cooling for Passive Houses
The full guide: split systems, bulkheads, hydronic and ducted reverse-cycle compared
Blower Door Testing
Get a measured ACH50 result to base your load calculation on, instead of an assumed one
What Is MVHR?
Why ventilation and heating/cooling are separate systems with separate jobs
ACH Calculator
Turn your blower door result into ACH50 and compare it to Australian benchmarks
What Is Passive House?
The building standard behind the lowest heating and cooling loads achievable
Home Performance Assessment
A proper load calculation based on your actual plans and measured airtightness
Frequently asked questions
What size air conditioner does a Passive House or airtight home need?
Much smaller than most people expect — often only a few kilowatts for an entire home, rather than the 10-16kW ducted systems common in conventional Australian construction. The exact figure depends on floor area, airtightness (ACH50), insulation level, glazing, orientation, shading and local climate. This calculator gives a rough estimate; a proper room-by-room load calculation gives the definitive number.
Why do installers often recommend a bigger system than I need?
Most conventional sizing rules of thumb were developed for older, leakier, less insulated homes and are usually based on floor area alone. Applied to an airtight, well-insulated building, they significantly overstate the actual heating and cooling load — leading to an oversized, less efficient system.
What's wrong with an oversized air conditioner?
An oversized unit short-cycles (turning on and off more often, which is harder on the equipment), dehumidifies poorly because it reaches temperature before it's run long enough to remove moisture, runs noisier, operates less efficiently at partial load, and costs more to buy than a correctly sized system for the same job.
How accurate is this calculator?
It's a simplified, single-zone estimate intended for education and rough planning — not a replacement for a proper heating and cooling load calculation. It uses standard heat-loss and heat-gain principles, but doesn't account for your specific floor plan, room-by-room distribution, exact glazing schedule, thermal bridging or a measured (rather than assumed) airtightness result.
Does my measured ACH50 matter more than an assumed one?
Yes. Ventilation and infiltration losses scale directly with your airtightness result, and a measured blower door result is far more reliable than an assumed value from drawings or building stage. Getting this number right — ideally mid-construction, before linings go on — is one of the most valuable inputs into an accurate load calculation.
This estimate is a starting point, not a design.
It’s here to give you a realistic order of magnitude before you talk to an installer — so you can recognise a wildly oversized quote when you see one. A proper heating and cooling design, based on your actual plans, glazing schedule and a measured airtightness result, is the only reliable way to size a system precisely.
Want a proper heating and cooling design?
HiPer Haus carries out home performance assessments across Adelaide and South Australia — combining a measured blower door result with a full load calculation based on your plans, insulation, glazing and airtightness target.
Written by
Jonathen HindryFounder of HiPer Haus. 25+ year plumber turned Certified Passive House Tradesperson — blower door testing, MVHR design and heat pump hot water across Adelaide and South Australia.
