What your NatHERS or HERS rating assumes about how you'll live in the house

Last month I sat in on an assessment for a client near Ballarat, a couple who'd just bought a 1970s brick veneer and wanted to know why the seller's disclosed 3.5-star rating felt generous. Within twenty minutes the assessor had answered the mystery, but not in the way the couple expected. It wasn't the insulation, or the single glazing, or even the leaky evaporative cooling ducting in the ceiling. It was the assumptions baked into the rating tool itself, assumptions about how people live in houses that have nothing to do with how this particular couple actually lived.

This is the part of a home energy rating almost nobody explains properly, and it's worth understanding before you either trust a star rating too much or dismiss it as meaningless. The rating isn't measuring your household. It's measuring a hypothetical, standardised household, and the gap between that hypothetical and your real life is where a lot of confusion starts.

The standardised occupant behind every NatHERS star rating

NatHERS software, whether it's AccuRate, BERS Pro or FirstRate5, runs a thermal simulation using a fixed occupancy schedule. It assumes set hours for when heating and cooling switch on, a defined internal temperature band the household is trying to hold, standard occupancy density per bedroom, and generic internal heat gains from appliances and people. None of this is measured from your family. It's a reference occupant, applied identically whether the house will hold a retired couple who are home all day or a share house of shift workers who are barely there.

The US HERS Index, administered under RESNET's protocols, works on a similar logic but expresses it differently. Rather than a star count, HERS produces an index score relative to a reference home built to a baseline code, again using standardised occupancy and equipment assumptions rather than the specific family moving in. Both systems exist to let you compare the fabric of one house against another on equal terms. That's the entire point, and it's a reasonable one. But it means the number on your certificate describes a modelled house, not your household's actual gas and electricity bills.

Why this modelling choice exists at all

If ratings used real occupant behaviour, you couldn't compare two houses fairly. A frugal retiree who never turns the heater past 18 degrees would make a poorly insulated house look efficient, and a family that runs the aircon at 20 degrees all summer would make a well-built home look wasteful. Standardising the occupant strips behaviour out of the equation so the rating reflects the building envelope, the glazing, the orientation, the insulation, the airtightness. That's genuinely useful information and it's the reason a NatHERS certificate is required at planning stage in most Australian jurisdictions and why HERS raters in the US work off RESNET's published standard rather than interviewing the future occupants about their thermostat habits.

I'd argue this is one of the more sensible parts of the whole system, honestly. The alternative, rating houses partly on the guessed behaviour of whoever happens to live there, would make the certificate almost useless for comparison purposes. The trade-off is that homeowners often read a star rating as a promise about their power bill, and it was never built to be that.

What the assumed occupancy schedule actually covers

The specifics matter here because they explain some genuinely odd outcomes. The standard schedule assumes heating and cooling are used to maintain comfort during occupied hours in living areas and bedrooms, with different assumed occupancy patterns for those two zone types. It assumes a fixed number of occupants scaled to bedroom count, not to how many people actually live there. It assumes generic hot water use patterns and appliance heat loads. None of these track your actual habits, whether you work from home, whether you're out at Hastings Lane's cafe strip for half your evenings, or whether you keep the whole house at 16 degrees because you grew up somewhere colder and don't feel the cold like everyone else does.

This is also why two houses with identical star ratings can have wildly different real-world running costs once occupied. The rating held behaviour constant and measured the envelope; your bill reflects behaviour layered on top of the envelope. If you want to understand why a high star rating still leaves you cold in winter, or why two assessors can land on different numbers for the same house, we've covered both of those separately, and the standardised occupant assumption threads through each explanation.

Where this trips up buyers and renovators

The most common mistake I see is someone using the star rating as a proxy for expected energy bills when comparing properties. It's not designed for that job, not directly. A 6-star home with excellent glazing but poor solar access might use less heating and cooling energy under the standard schedule than a 7-star home with different envelope characteristics, yet the real occupants of either house could flip that outcome entirely depending on their actual habits, appliance choices and hot water system.

The second trip-up is renovators assuming that fixing the fabric alone will translate directly into proportional savings on their bill. If your actual usage pattern is nothing like the standardised schedule, say you run heating fifteen hours a day because you're home with young kids, or barely at all because you travel for work, your realised savings from an insulation or glazing upgrade will scale differently than the modelled improvement in your star rating suggests. The rating tells you the building got better. It doesn't tell you by how much your specific bill will move.

Assessors I've worked alongside, including one in regional Victoria who's been doing this since the tool was called NatHERS 2.0, are pretty upfront about this when asked directly. The honest answer is always some version of: the certificate rates the house, not you.

What actually varies between the assumed and the real

A few things sit outside the standardised model entirely and are worth knowing about if you're trying to bridge the gap between rating and reality. Appliance efficiency and choice aren't part of the thermal envelope calculation in the way you'd expect; a heat pump versus a resistive electric heater changes your actual energy consumption dramatically while barely touching your star rating, because the rating is measuring the thermal load, not the plant efficiency used to meet it. Internal heat gains from a household running four laptops and a home office all day differ from the generic assumption too, sometimes helping in winter and hurting in summer.

Orientation-specific behaviour is another gap, and it compounds the standardised-occupant issue rather than sitting separately from it. We've written before about how these ratings treat orientation as a fixed input rather than something you might actively manage, closing blinds on the western side in summer, opening things up on a mild autumn evening. The standard schedule doesn't reward or penalise that kind of active occupant behaviour at all.

None of this means the rating is broken. Both NatHERS, overseen by the federal Department of Climate Change, Energy, the Environment and Water, and the RESNET HERS standard used across most of the US, are calibrated tools designed for a specific comparative job. Understanding what that job is, and isn't, is the difference between using the number well and being disappointed by it.

How to use the number without over-trusting it

Treat the star rating or HERS index as a fabric scorecard, not a bill forecaster. When comparing two properties, look past the headline number to what's driving it, glazing spec, insulation levels reported in the certificate, and airtightness where that's been assessed. If you want a clearer read on your likely running costs, that comes from combining the fabric rating with your actual planned equipment and your honest read on your own household's habits, not from the certificate alone. Our piece on what a NatHERS or HERS assessor actually checks on-site is a good companion read if you're heading into an assessment and want to know what's actually being measured versus assumed.

If you're renovating specifically to chase a higher number for compliance reasons, that's a legitimate goal, but don't confuse it with a comfort or bill-reduction plan on its own. We've written about why a rating won't tell you what to fix first, and the standardised occupancy assumption is part of why: the modelled priorities and your lived priorities can genuinely diverge. Pairing the certificate with a straightforward look at air sealing before insulation and your glazing spec, per our double vs triple glazing breakdown, gets you further than staring at the star count alone.

A quick reality check before you rely on the certificate

Ask your assessor, or the seller's disclosed certificate, what heating and cooling setpoints and hours were assumed, and compare that honestly against how your household actually lives. If you're a family that's home all day with the heater on from 6am, your real energy use for a 6-star house could sit well above what a 7-star house achieves for a household that's out most of the day. Neither number is wrong. They're just answering a different question than "what will I pay."

For background on how the two major systems compare structurally, our explainer on NatHERS vs HERS covers the methodology differences in more depth, and NatHERS's own technical documentation, published by the Department of Climate Change, Energy, the Environment and Water, lays out the exact assumed schedule if you want the primary source. RESNET's published HERS standard does the same for the US index. Reading either is less painful than it sounds and clears up most of the "why does my bill not match my rating" confusion in one sitting.

Back at that Ballarat inspection, once the couple understood the standardised occupant assumption, the 3.5-star figure made a lot more sense. It wasn't a lie about the house. It was an honest answer to a slightly different question than the one they'd been asking.