What Size Split System Do I Need?

How Are Split System Sizes Measured?

Understanding how split system sizes are measured is essential for any homeowner. In Australia, sizes are primarily rated in kilowatts (kW), which indicates the cooling or heating capacity. For reference, 1 kW equals about 3,412 British Thermal Units (BTU) per hour, a measurement more common in the US but sometimes referenced here.

kW ratings tell you how much heat the unit can remove (cooling) or add (heating) to a space. For example, a 2.5 kW system is suitable for small rooms, while larger areas might need 5 kW or more. Always check both cooling and heating capacities if you’re opting for a reverse-cycle model, as they’re often listed separately.

BTUs are occasionally used in product specs, especially for imported models. To convert, multiply kW by 3,412—for instance, a 3.5 kW unit is roughly 12,000 BTU. When shopping, look for Energy Star ratings alongside these measurements to ensure efficiency.

This knowledge empowers you to compare models effectively and ask the right questions when consulting professionals.

How to Choose Split System Size: A Step-by-Step Guide to Calculation

Figuring out what size split system you need doesn’t have to be overwhelming. Follow this step-by-step split system sizing guide to estimate your requirements. While online calculators can help, understanding the process ensures accuracy.

Step 1: Measure Your Room Dimensions

Start by calculating the room’s area in square metres (length x width). Don’t forget ceiling height—standard is 2.4 metres, but higher ceilings (e.g., 3 metres) increase volume and may require 10–20% more capacity.

Step 2: Assess Insulation Quality

Well-insulated homes retain temperature better, reducing the needed kW. Check for wall, ceiling and floor insulation. Poor insulation? Add 10–15% to your estimate. In older Australian homes, especially in regions like Queensland, upgrading insulation can make a big difference.

Step 3: Evaluate Windows and Sun Exposure

Windows let in heat, especially those facing north in sunny climates. Count the number and size of windows, and note if they’re double-glazed or shaded. High sun exposure? Increase capacity by 10–20%. Use curtains or blinds to mitigate this during peak hours.

Step 4: Factor in Appliances and Heat Sources

Appliances like ovens, computers, or lights generate heat. For a kitchen, add 0.5–1 kW if it includes cooking appliances. In home offices, account for multiple devices, each computer might add 0.1–0.2 kW.

Step 5: Consider Occupancy

More people mean more body heat. Add about 0.6 kW per additional person beyond two in the room. For family gatherings in living areas, this can significantly impact sizing.

Step 6: Use a Basic Formula

A rough rule: 0.125–0.15 kW per square metre for cooling in moderate climates. Multiply your room area by this factor, then adjust for the above elements. For example, a 30 sqm room might need a 3.75–4.5 kW base, plus extras for poor insulation.

Tools like the Daikin or Mitsubishi sizing calculators can refine this, but always verify with a pro.

What if Your Split System is Undersized?

An undersized split system struggles to keep up, leading to several issues. It runs constantly, trying to reach the desired temperature, which spikes energy bills and wears out components faster. In humid Australian summers, this means poor dehumidification, leaving rooms feeling sticky.

Temperatures fluctuate and the unit can’t handle heatwaves effectively. Over time, this constant strain can lead to breakdowns, costing you in repairs. If you’re asking “What size split system do I need?” to avoid these pitfalls, erring on the side of slight oversizing (with professional input) is often better than going too small.

What if Your Split System is Oversized?

An oversized unit cools or heats too quickly, causing short cycling—frequent on-off switches. This inefficiency wastes energy and increases running costs, as the system doesn’t run long enough to dehumidify properly.

Humidity builds up, promoting mould growth in damp-prone areas like bathrooms or basements. The constant starting also stresses the compressor, shortening its lifespan. In Australia, where reverse-cycle systems are popular, oversizing can make heating uneven, with rooms feeling drafty. Proper sizing prevents these hidden expenses.

Sizing Guidance by Room

Since each room has distinct requirements, customise your strategy accordingly.

Best Split System Size for Bedroom

For bedrooms, aim for quiet, efficient cooling. A 2.0–3.5 kW unit suits most (up to 25 sqm), ensuring restful sleep without noise. 

Sizing for Living Rooms

Living rooms, often 30–50 sqm, require 3.5–5.0 kW. Consider entertainment systems and frequent use. Open-plan? Treat as one space, potentially needing 5–7 kW for even distribution.

Open-Plan Spaces

These larger areas (45+ sqm) benefit from 5–8 kW systems or multi-split setups. High ceilings and multiple zones increase demands, a professional zoning advice is key for efficiency.

Home Offices

With computers and lighting, home offices (15–25 sqm) need 2.5–3.5 kW. Ensure quiet operation for productivity. In sunny spots, prioritise models with good airflow control.

When to Consult an Expert for Split System Sizing?

While this guide helps estimate what size split system you need, nothing beats a professional assessment. HVAC experts use tools like heat load calculations to account for your home’s specifics, including climate zone (e.g., Brisbane’s humidity vs Melbourne’s chill).

They’ll inspect insulation, ducting (if applicable), and energy efficiency, recommending brands like Fujitsu or Panasonic. In Australia, licensed installers ensure compliance with standards, potentially qualifying you for rebates. Don’t risk DIY errors, contact a local specialist for peace of mind.