Two NatHERS Ratings Explained
The NatHERS thermal star rating and Whole of Home rating measure different parts of residential energy performance. One assesses the thermal response of the building shell. The other assesses major household energy uses, onsite generation and storage.
NatHERS and Whole of Home are sometimes described as competing assessment methods. This is no longer an accurate way to understand their relationship.
NatHERS is the wider residential energy-rating framework. Within that framework, a proposed home can receive a thermal star rating out of 10 and a separate Whole of Home rating out of 100. The two results answer different questions about the same dwelling.
The thermal star rating asks how much heating and cooling the home is likely to need because of its design and construction. The Whole of Home rating builds on those thermal loads and considers the energy performance of household systems, solar generation and battery storage.
In Brief
Two Ratings, Two Different Outcomes
Thermal Star Rating
A rating out of 10 that assesses the modelled heating and cooling needs created by the home’s design, construction and climate.
Whole of Home Rating
A separate rating out of 100 that assesses major household energy uses together with onsite solar generation and battery storage.
Combined View
Together, the ratings describe the thermal performance of the building shell and the wider modelled energy performance of the dwelling.
Whole of Home does not replace NatHERS. It is an additional NatHERS rating that complements the thermal star rating and may support the annual energy-use component of the applicable residential energy pathway.
Building-Shell Performance
What Does the NatHERS Thermal Star Rating Measure?
The NatHERS thermal star rating assesses how the design and construction of a dwelling influence the energy needed to maintain comfortable indoor temperatures.
Accredited software models the home in its local climate and estimates its annual heating and cooling loads. The result is converted to a rating between 0 and 10 stars.
Important thermal-rating inputs include:
The star rating does not directly assess the efficiency of the proposed air conditioner, hot-water unit, cooking equipment, solar system or battery. Those systems belong to the Whole of Home calculation.
Household Energy Performance
What Does the Whole of Home Rating Measure?
The NatHERS Whole of Home rating assesses the wider modelled energy performance of the dwelling. It uses the heating and cooling loads established by the thermal model and adds the energy performance of major household systems.
The assessment considers:
The result is expressed as a rating out of 100. A score of 100 represents a net zero energy-value home under the NatHERS calculation method, while ratings above 100 are possible where onsite generation exceeds the assessed energy value of the dwelling.
The score is not a percentage and is not a direct prediction of the future household energy bill. It is produced using standardised occupancy, equipment-use and operating assumptions so that dwellings can be compared consistently.
Side-by-Side Comparison
Thermal Star Rating vs Whole of Home Rating
Building-Shell Rating
Thermal Star Rating
Household Energy Rating
Whole of Home Rating
Connected Assessments
How Do the Two NatHERS Ratings Work Together?
The Whole of Home calculation does not begin with an entirely separate dwelling model. It uses the heating and cooling loads produced through the thermal assessment and then applies the performance of the proposed household systems.
A better-performing thermal shell generally reduces the amount of heating and cooling energy carried into the Whole of Home calculation. The final Whole of Home result will also depend on the type and efficiency of the equipment selected and the contribution of onsite generation and storage.
Strong Shell, Weak Systems
A home may achieve a strong star rating but receive a weaker Whole of Home result if inefficient heating, cooling or hot-water equipment is specified.
Weak Shell, More Solar
Solar generation may strengthen the Whole of Home score, but it does not improve the separate thermal star rating of the building shell.
Coordinated Design
A balanced outcome coordinates the building shell, efficient systems and onsite energy rather than relying on one late intervention.
The ratings should therefore be reviewed together. One explains the thermal demand created by the dwelling, while the other shows how that demand interacts with household systems and onsite energy.
Assessment Documentation
How Do the Results Appear on a NatHERS Certificate?
A NatHERS certificate records the dwelling assessed through accredited software and identifies the plans, construction information and modelling inputs associated with the result.
Where both assessments have been completed, the certificate can display:
If a Whole of Home assessment has not been completed, the certificate can indicate that no Whole of Home rating was undertaken.
The project drawings and specifications should remain consistent with the glazing, insulation, construction and household systems represented in the assessment. Material changes may require the model and certificate to be reviewed.
Rating Influences
Which Decisions Affect Each Rating?
Both Ratings
Thermal Design
Orientation, glazing, insulation, shading and construction affect the star rating and influence the heating and cooling loads used in Whole of Home.
Whole of Home
Heating and Cooling Systems
Equipment type, efficiency, energy source and the areas served affect the Whole of Home result rather than the thermal star rating itself.
Whole of Home
Hot-Water System
The selected system type, fuel and performance characteristics can materially influence the household energy rating.
Whole of Home
Cooking and Household Loads
Cooking energy, lighting and standardised plug loads contribute to the Whole of Home calculation.
Whole of Home
Solar PV
Onsite solar generation can offset modelled household energy demand and improve the Whole of Home score.
Whole of Home
Battery Storage
Where included, battery capacity and its relationship with solar generation contribute to the assessed energy outcome.
Common Misunderstandings
What the Two Ratings Should Not Be Confused With
Whole of Home is not outside NatHERS. It is a complementary rating within the wider NatHERS framework.
The star rating is not an appliance rating. It models the thermal loads created by the building shell.
A Whole of Home score is not a percentage. A result of 80 does not mean the dwelling is 80 per cent efficient.
Solar does not improve the thermal star rating. Solar PV is accounted for within the Whole of Home calculation.
A high star rating does not guarantee a high Whole of Home score. Household-system efficiency and onsite energy still affect the final result.
The ratings are not energy-bill guarantees. Actual household consumption depends on occupancy, behaviour, weather, tariffs and equipment operation.
Residential Energy Requirements
When May Both Ratings Be Required?
NCC 2022 introduced separate residential requirements for thermal performance and annual energy use. Where the NatHERS pathway applies, the thermal star rating may address the thermal-performance requirement and the Whole of Home rating may address the annual energy-use requirement.
The exact assessment requirement depends on the project location, building classification, approval pathway and the NCC provisions adopted by the relevant state or territory. Jurisdictional variations and transitional arrangements may affect what is required.
New South Wales projects generally follow the BASIX pathway. BASIX may use NatHERS thermal modelling and considers related household energy systems, but a BASIX Certificate and a NatHERS Whole of Home rating are not interchangeable documents.
For more detail on the NSW relationship, read BASIX and NatHERS Whole of Home: How Do They Work Together in NSW?
Design Coordination
Why Should the Ratings Be Considered Early?
The thermal star rating and Whole of Home rating are easier to coordinate while the building design and household systems can still be adjusted.
An early assessment can identify whether the thermal shell is creating excessive heating or cooling loads and whether the proposed heating, cooling, hot-water, solar or battery systems support the required Whole of Home result.
Possible responses can then be compared before documentation is fixed. These may involve changes to glazing, insulation, shading, equipment efficiency, system selection or onsite generation.
When modelling is delayed until the end of design, the remaining options may be more limited or require wider changes across the architectural and services documentation.
Frequently Asked Questions
NatHERS Star Rating and Whole of Home FAQs
Is Whole of Home separate from NatHERS?
No. Whole of Home is a complementary rating within the NatHERS framework. It works alongside the thermal star rating rather than replacing NatHERS or operating as an unrelated assessment scheme.
What is the difference between the two ratings?
The thermal star rating assesses the heating and cooling loads created by the home’s design and construction. The Whole of Home rating assesses major household energy uses together with onsite solar generation and battery storage.
Is the Whole of Home score included in the star rating?
No. The thermal star rating and Whole of Home rating are separate results. They may appear together on the NatHERS certificate, but each uses its own scale and assessment purpose.
Can solar panels improve the thermal star rating?
No. Solar PV contributes to the Whole of Home calculation but does not change the thermal star rating. The star rating is determined by the thermal performance of the building design and construction.
Can a high star rating improve the Whole of Home result?
It can help. A stronger thermal design generally reduces modelled heating and cooling loads. The Whole of Home result will still depend on the household systems, standardised loads, onsite generation and battery storage included in the assessment.
What does a Whole of Home score of 100 mean?
A score of 100 represents a net zero energy-value home under the NatHERS Whole of Home calculation method. Scores above 100 are possible where the modelled benefit of onsite generation exceeds the assessed energy value of the dwelling.
Does the Whole of Home score predict the future energy bill?
No. The rating uses standardised assumptions so that homes can be compared consistently. Actual household consumption and costs will vary with occupancy, behaviour, weather, tariffs and equipment operation.
Are both ratings always required?
Not in every project context. The requirement depends on the building classification, jurisdiction, approval pathway and the residential energy provisions applying to the development.
Can assessment begin before all systems are selected?
Yes. Preliminary assessment can use documented assumptions or provisional specifications. The model should be updated as glazing, construction and household systems are confirmed so the final certificate reflects the project intended for approval and construction.
What happens if the design changes after certification?
Changes to the plans, glazing, insulation, construction, heating and cooling equipment, hot-water system, solar PV or battery storage may affect one or both ratings. Relevant changes should be reviewed by the assessor before the final documentation is relied upon.
Related Guidance
Explore the Two NatHERS Rating Pathways
Residential Energy Assessment
Need to Confirm Which NatHERS Ratings Apply?
Certified Energy can review the available plans, project location, construction information and proposed household systems to clarify the thermal and Whole of Home assessment requirements.
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