Last reviewed: 1 August 2026 by the Ozbuyz Store Editorial Team.
Quick answer: Do not size a heater from floor area alone. Measure the room’s length, width and ceiling height, then account for insulation, glazing, draughts, outdoor winter conditions and whether the space is enclosed. Use manufacturer room guidance for portable heaters and qualified sizing advice for installed systems.
The old shortcut of applying one watts-per-square-metre number to every room ignores too many variables. A compact, insulated bedroom in coastal Sydney and a room of the same floor area with high ceilings and single glazing in inland NSW can have very different heat loss.
Step 1: measure floor area and volume
Record:
- Floor area: length × width in metres.
- Room volume: floor area × average ceiling height.
- Open connections: hallways, stairwells, kitchens or adjoining rooms that cannot be closed.
Room volume helps explain why a high-ceilinged room can need more heating than another room with the same floor area.
Step 2: assess heat-loss factors
| Factor | Why it matters | What to record |
|---|---|---|
| Climate and exposure | Colder outdoor temperatures and wind increase heat loss | Location, exposed walls and prevailing winter wind |
| Insulation | Roof, wall and floor insulation slow heat transfer | Known insulation, roof space, slab or suspended floor |
| Windows and glazing | Large or single-glazed windows can lose substantial heat | Window area, glazing type, curtains or blinds |
| Draughts | Uncontrolled air leakage replaces warmed air with cold air | Gaps around doors and windows, unused vents or chimneys |
| Ceiling height | More air volume must be heated | Standard, high, raked or double-height ceiling |
| Zoning | Open doors and connected spaces enlarge the effective load | Whether the room can be closed during use |
Step 3: decide what “heated” means
There is a major difference between taking the chill off one occupied corner, maintaining a comfortable closed bedroom and heating an open living zone for hours. A radiant heater may make a seated person comfortable without heating every cubic metre of air. A convection heater must compensate for the room’s ongoing heat loss.
Step 4: compare manufacturer guidance
For portable heaters, review the rated input, the manufacturer’s recommended room guidance, thermostat and intended use. Treat advertised room coverage as an estimate, not a guarantee, because test conditions may not match your home.
For reverse-cycle air conditioning, use the capacity and climate-zone information on the Zoned Energy Rating Label, then obtain sizing advice that considers construction, windows and local climate. Australian Government guidance warns that both undersized and oversized air conditioners can cost more and perform poorly.
Step 5: check the electrical and practical limit
Do not solve a difficult room by adding unplanned high-load appliances. Confirm the product’s electrical requirements, use one appliance per power point, and have questionable outlets or circuits assessed by a licensed electrician. A large open-plan room may be better served by an installed system, zoning or building-envelope improvements than multiple portable heaters.
A worksheet to take shopping
- Room length: ___ m
- Room width: ___ m
- Average ceiling height: ___ m
- Floor area: ___ m²
- Room volume: ___ m³
- Can the room be closed? Yes / No
- Window area and glazing: ___
- Insulation known? ___
- Draughts or open chimney? ___
- Typical hours of use: ___
- Local winter conditions: ___
- Need whole-room or personal heat? ___
Improve the room before increasing heater size
YourHome guidance recommends insulation, suitable glazing, close-fitting curtains or blinds and draught sealing to reduce heat loss. Use removable seals carefully and never block required ventilation, especially where combustion appliances are present.
Frequently asked questions
How many watts per square metre do I need?
There is no universal figure that is reliable for every Australian room. Floor area is only a starting measurement; ceiling height, climate, insulation, glazing, draughts and open connections can materially change the load.
Should I buy the biggest portable heater available?
No. Larger input increases maximum electricity use and does not fix poor zoning or high heat loss. Choose a heater suited to the job and room, with a usable thermostat and safe electrical connection.
Why does my correctly rated heater still struggle?
The room may have more volume, glazing, draughts or open connections than the rating assumes, or the heater type may not suit the task. Check the building envelope and manufacturer guidance before adding more equipment.
Who should size a reverse-cycle air conditioner?
A qualified supplier or installer should assess the space, construction and climate. Compare the resulting options using the Zoned Energy Rating Label for your climate zone.
Research and review method
This guide replaces a generic 100 W/m² rule with the factors identified in Australian Government Energy Rating and YourHome guidance. It provides general buying information, not a heat-load calculation or electrical advice.