When Numbers Start Making Decisions · Season Three, “The People the Average Does Not See” · Article 11
1. Seven stars describe modelled performance, not a future bill
A buyer moves into a new seven-star home and expects low energy bills. The first year is expensive. Heating and cooling run for long periods, several people work from home, appliances consume more than expected, and an extreme season tests the building. Was the rating wrong?
Not necessarily. The Nationwide House Energy Rating Scheme uses accredited software to model how much heating and cooling a home needs to remain comfortable under standard assumptions and its local climate. A seven-star home should require relatively little heating and cooling compared with a lower-rated design, and seven stars is the minimum thermal-performance standard for new houses under the National Construction Code 2022 pathway. NatHERS: Home Energy Rating Certificate
The star is principally evidence about the building shell and design under a common model. Actual energy use also depends on construction quality, occupants, appliance efficiency, energy source, weather, tariffs and the comfort they seek.
The distinction resembles a vehicle fuel test: standard conditions make designs comparable, while real use remains variable. The rating is valuable because it is not a prediction tailored to one unknown household. It becomes misleading only when marketed as if it were one.
2. Standard occupancy makes comparison possible
Design-stage rating requires assumptions about thermostat settings, hours of occupancy, internal heat from people and appliances, room use and weather. If every project could choose a favourable occupant, ratings would not be comparable. NatHERS therefore specifies internal heat loads and operating assumptions for accredited software. NatHERS: Internal heat loads
These assumptions produce a standard test household. They do not claim that every Australian family lives that way. A single retiree at home all day, a large family, shift workers and an empty investment property interact differently with the same building.
Standardisation isolates design performance. Better orientation, insulation, glazing, sealing and shading should reduce the modelled energy needed for comfort. Without a common model, a poorly designed home could appear efficient merely because an imagined occupant tolerated uncomfortable temperatures.
The model average is therefore deliberate. Fair communication should say what it standardises and which later differences it leaves to occupants and operation.
3. Thermal stars and whole-of-home performance are related but different
NatHERS began with thermal performance: the heating and cooling energy load associated with the building shell. Whole-of-home assessments add fixed appliances and on-site energy generation to estimate broader household energy performance. The certificate indicates which assessment has been completed.
A strong thermal shell can coexist with inefficient hot water, pool equipment, lighting, cooking and plug-in appliances. Conversely, solar generation can reduce net purchased energy without correcting poor thermal design. Combining the two into a vague claim of “energy efficient” hides which part was measured.
NatHERS explains that whole-of-home ratings build on the star rating by adding appliances and other information. About NatHERS
Buyers should therefore ask whether “seven stars” refers to thermal performance, whether a whole-of-home rating exists, what appliances were specified and whether solar was included. The number is meaningful only with its boundary.
4. The built house can differ from the rated design
A design is assessed from plans and specifications. Real construction introduces gaps, substitutions and workmanship. Insulation can be compressed or omitted around services. Sealing can be incomplete. Glazing or colours can differ from the specification. Shading and orientation can be altered during construction.
If the delivered building does not match the rated design, unexpectedly high heating and cooling needs may reveal a compliance or quality problem rather than unusual occupants. Documentation, inspections, product records and, where appropriate, airtightness or thermal testing can reconnect the certificate to the physical house.
The National Construction Code’s 2022 energy-efficiency changes raised residential requirements and combined thermal performance with a whole-of-home energy budget pathway. ABCB: Overview of changes to energy efficiency and condensation
A rating system should not stop at calculation. The more it influences sale price and compliance, the stronger the need to verify that what was modelled was built.
5. Comfort is an outcome, not a fixed behaviour
Households use energy partly to obtain comfort. People differ in temperature sensitivity, health needs, clothing, mobility and time at home. Heatwaves and cold snaps increase demand. A person with a medical condition may need a narrower temperature range than the standard model assumes.
High actual energy use can therefore reflect waste, a construction defect, extreme weather or the legitimate production of safety and comfort. Bills alone cannot distinguish them. Criticising occupants for “using the house incorrectly” may place the burden of model mismatch on those least able to tolerate heat or cold.
The correct counterfactual is often not zero energy. It is how much energy the same household would have needed in a lower-performing building under the same conditions. That quantity is difficult to observe, which is why the design model remains useful.
Equity policy should combine efficient buildings with affordable energy and accessible controls. A seven-star standard reduces expected need but does not guarantee that every household can afford the remaining need.
6. Post-occupancy evidence should be allowed to correct the design system
Smart-meter data, indoor temperature, occupant feedback and construction inspections can reveal a performance gap. The analysis should normalise for weather and occupancy where possible instead of comparing raw bills alone.
Patterns matter. If many homes of one design use more heating and cooling than modelled, software assumptions or construction practices may need review. If one home is an outlier, faults, settings and household circumstances should be investigated. If vulnerable occupants systematically need more energy than assumed, policy should treat that as distributional evidence.
Privacy is important. Detailed energy data can reveal occupancy and behaviour. Post-occupancy evaluation should collect only what is needed, obtain appropriate consent and report aggregated results.
The feedback loop gives the star rating institutional humility. A model is trustworthy not because it can never be contradicted, but because evidence from lived homes can improve its next version.
7. A practical framework for building ratings
When a star rating is used to describe a home, ask:
- Is it a thermal star rating or a whole-of-home result?
- Which climate file and standard occupancy assumptions were used?
- Does the certificate match the final design and construction?
- Which appliances, generation and plug loads are outside the score?
- How do actual occupants and comfort needs differ from the model?
- Has unusual weather been considered?
- Can buyers access the certificate and understand its scope?
- Are post-occupancy performance gaps audited and fed back into standards?
These questions let the rating do its proper work: compare designs under common conditions and guide better construction.
Conclusion: seven stars are a design promise with stated conditions
NatHERS makes the invisible thermal consequences of plans visible before a home is built. A seven-star requirement encourages better orientation, insulation, glazing and sealing and should reduce the energy needed for comfort relative to a weaker design.
My judgement is that the rating should be presented as modelled design performance, supported by construction verification and post-occupancy learning. It should not be advertised as a guaranteed bill or used to blame occupants whose health, schedules or comfort needs differ from the standard household.
The model home is necessary to compare buildings. The lived home begins when particular people move in.
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