Design Considerations / Comfort & Energy / Start here
What do seven stars measure?
A seven-star rating is a prediction. Software runs the house through a year of local weather and works out how much heating and cooling it would need to keep a standard household comfortable. The less energy it needs, the more stars it earns.
Design ConsiderationsComfort & EnergyAt a glance
A star rating predicts how much heating and cooling a house will need to stay comfortable in its own climate. It rates the building, meaning its walls, windows, insulation and orientation. It doesn't rate the appliances or the people living in it.
- Seven stars is the minimum for new homes in the ACT, and the thermal standard for most new homes in NSW.
- Stars are relative to the local climate. A seven-star Canberra house can need four times the heating and cooling of a seven-star Sydney house.
- In Canberra, going from six stars to seven cuts heating and cooling by about a quarter.
- The rating is per square metre, so a big seven-star house can still use more energy than a small six-star one.
- Winter and summer loads are capped separately, so a design can't pass by doing well in one season only.
- Whole of Home is a second, separate rating for appliances and solar. Good equipment can't make up for a poorly designed building.
- The cheapest stars come from decisions at sketch stage: which way the house faces, how big the windows are and where they go, and how deep the eaves are.
The calculationA year of weather, hour by hour
A NatHERS assessor builds a model of the house from the drawings and specifications. The model includes every room, wall, window, floor and roof, the materials and insulation in each, and the eaves, fences and neighbouring buildings that shade it. Accredited software, built on the CSIRO's Chenath engine, then runs the house through a reference year of weather for its location. Australia is divided into 69 NatHERS climate zones, and Canberra is zone 24. The software works out the temperature in every room for each of the 8,760 hours in that year.
The house is tested with a standard household living in it:
- Living areas, including the kitchen, are heated to at least 20°C from 7 am to midnight.
- Bedrooms are heated to 18°C from 7 to 9 am and from 4 pm to midnight, and to 15°C overnight, on the assumption that people are under blankets.
- Cooling starts at a temperature set for each climate zone, allowing for humidity and air movement.
- Bedrooms are left alone through the middle of the day, and living areas overnight.
- Windows are opened and blinds adjusted whenever that helps. Cooking, lighting and people add heat, most of it around the evening meal.
The energy needed to hold those temperatures is the house's thermal load. It is added up over the year and divided by the floor area, giving a figure in megajoules per square metre (MJ/m²). Each climate zone has its own scale for turning that figure into a rating from zero to ten stars.
Because the result is per square metre, size doesn't count against a house. Using the Canberra scale below, a 350 m² seven-star home needs about 43,000 MJ of heating and cooling a year. A 150 m² six-star home needs about 24,000 MJ. If running costs matter to you, the size of the house matters as much as its stars.
The scaleSeven stars in Canberra is not seven stars in Sydney
Each climate zone has its own star scale, so the stars measure how well a house suits its own climate. They don't measure energy in absolute terms.
| Annual heating and cooling allowed (MJ/m²) | 6 stars | 7 stars | 10 stars |
|---|---|---|---|
| Canberra (zone 24) | 161 | 122 | 18 |
| Sydney (zone 17) | 39 | 30 | 7 |
| Cabramurra, Snowy Mountains (zone 25) | 366 | 298 | 119 |
A seven-star Canberra house can need four times the heating and cooling of a seven-star Sydney house and still earn the same rating. Canberra's winters set the scale. A star rating is only useful for comparing houses in the same place.
Within one place, each star is a real step. In Canberra, going from six stars to seven cuts the allowed load from about 161 to about 122 MJ/m², which is roughly a quarter less heating and cooling. Ten stars allows almost none.
The zone comes from the site's postcode, so check that the certificate names the right one.
Two limitsWinter and summer are capped separately
A star rating adds heating and cooling together. In principle, a house could reach seven stars by being very good in winter and poor in summer. Since 2019 the National Construction Code has stopped that by setting separate limits on each load. They were tightened in 2022 to match the seven-star standard.
For a Canberra house, the limits depend on the floor:
| Canberra, 7 stars (MJ/m² a year) | Heating limit | Cooling limit |
|---|---|---|
| Slab on ground | 129 | 34 |
| Suspended floor | 123 | 41 |
The heating limit sits close to the total allowed for seven stars, so in practice the cooling limit is the one that bites. A design with large, unshaded north-west or west glazing can reach seven stars on paper and still fail because it overheats in summer. Canberra summers are short but hot, and the limit is there to make sure they're designed for.
The certificate reports heating and cooling loads separately. They're worth reading. Two seven-star houses can have quite different splits, and the split tells you where each house will be uncomfortable.
NSW sets its own caps through BASIX, so these NCC limits don't apply there.
Two ratingsStars and Whole of Home
Since NCC 2022, continued in NCC 2025, there has been a second test, and certificates carry two numbers.
The star rating is about the building: how much heating and cooling the shell will need. It assumes nothing about which heater or air conditioner gets installed.
The Whole of Home rating is about the equipment. It estimates the yearly energy used by the heating and cooling system, hot water, lighting, and any pool or spa pump, weighted for cost and emissions. Rooftop solar and batteries count against that use. The score runs from 0 to 100, where 100 means the house is expected to produce as much energy as that equipment uses over a year. Scores above 100 are possible. New houses need at least 60.
The two ratings reward different decisions. A heat pump hot water system, an efficient reverse-cycle system or a few more solar panels can lift the Whole of Home score quickly. None of them changes the star rating. Panels don't stop a west-facing bedroom overheating, and a better air conditioner only spends less energy fighting the same heat. Equipment wears out and gets replaced every fifteen years or so, but the walls, windows and orientation stay for the life of the house. Get the stars right first.
The standard household isn't yours
Real households behave differently. Across 40 western Sydney homes monitored for two years, people used their bedrooms during the day, and kept them warmer at night than the rating's 15°C. Modelled with those habits, the homes needed up to 26% more energy, which is roughly the difference of a full star. The rating is still a fair way to compare two designs, because both face the same assumptions. It can't tell you how your household will live in the result. If someone works from a bedroom, or you like the house warm, say so early. The design can allow for it even when the rating doesn't.
Design firstWhat moves the number
Most homes are rated once, at the end, to show the certifier they comply. By then the plan, structure and windows are fixed, and the only remaining options are upgrades: dearer glazing, more insulation, more shading. Used early, the same software is a design tool. It reports heating and cooling room by room, so it shows which spaces are causing the problem.
Modelling of typical houses under the older star bands shows the scale. The exact figures will differ under today's bands, but the trends hold:
- Orientation. The same house, rotated in 45° steps, ranged from 7.7 stars with its living areas facing north to 5.8 facing west. That's nearly two stars from orientation alone, at no cost.
- Ceiling insulation. Ratings rose steeply up to about R4.0 and then barely moved. Past a point, more insulation is money spent for nothing.
- Windows. Upgrading from single-glazed aluminium to double-glazed or low-E with thermally broken frames added about 0.8 of a star. Making a poorly placed window smaller was sometimes cheaper than upgrading the glass.
The cheapest stars come from decisions made at sketch stage: where the living areas sit, how big the windows are and which way they face, and how deep the eaves are. Because I prepare the ratings for my own designs, I test these choices while the plan can still change, not after it's been drawn up.
Applying it locallyCanberra and New South Wales
New homes in the ACT have had to meet the equivalent of seven stars, plus a Whole of Home score of at least 60, since January 2024. NCC 2025 carries both requirements forward with minimal change, and the ACT's version adds a Passive House pathway.
Existing homes in the ACT have to disclose an Energy Efficiency Rating (EER) when they're sold or leased. Canberra has required this since the late 1990s. The star rating has to appear in sale advertisements, and the EER statement goes with the contract. Most of these ratings are done with FirstRate 4, an older tool that rates the building shell only. An EER on a 1970s house isn't directly comparable with a NatHERS certificate on a new design. The market pays attention all the same. Between 2011 and 2016, seven-star homes in the ACT sold for up to 9.4% more than comparable three-star homes. Rental homes must also meet a minimum standard for ceiling insulation.
In NSW, BASIX sets the energy and thermal targets for new homes in place of the NCC rules, and it has its own heating and cooling caps. Since 1 October 2023 the BASIX thermal standard for most new homes has been seven NatHERS stars, up from an average of 5.5 to 6. BASIX also sets energy and water targets and, since that update, measures the embodied emissions of the building materials. A NatHERS assessment is the usual way to show thermal compliance.
The climates differ as much as the rules. The Southern Tablelands behave much like Canberra, with long heating seasons. On the coast, humidity and summer cooling take over. In the alpine areas, heating dominates everything. The house has to be designed for its own zone.
Check the code edition, and any BASIX requirement, for each project.