Thermarate

Payback lab · NatHERS star bands

Does the last star pay for itself?

A design that falls short of 7 stars under NatHERS gets an upgrade list. The upgrades cut the heating and cooling NatHERS models, and they’re paid for up front. This lab prices what they save, from NatHERS’s own star bands and WA’s household electricity rate, so the saving can be set against the cost.

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The arithmeticFour steps
  1. 01 · NatHERS star bandsThe most heating and cooling each rating allows in the climate zone, per m² a year
  2. 02 · The homeThe step from the rating as drawn to 7.0 stars, across its conditioned floor
  3. 03 · The air-conditionerThat heating and cooling as electricity, through the system’s efficiency
  4. 04 · The priceElectricity at WA’s household rate, added up year by year
Every figure is shown, and where the lab has to assume, it assumes in the upgrades’ favour.

01The payback lab

What reaching 7 stars saves a year.

Choose the place and the home, and the design’s rating as drawn. The lab reads NatHERS’s limits for that rating and for 7.0 stars in the zone, and prices the difference at WA’s household rate, year by year. Add the price of the upgrade list to see when, or whether, it pays for itself.

Sets the climate zone.

200 m²

The area NatHERS heats and cools: the home without its garage, porch or alfresco.

6.0 stars

From the assessor, before any upgrades. The lab prices the step to 7.0 stars.

Air-conditioner efficiency
Price rise a year
Over
c/kWh
$
Heating and cooling a year, by star rating
The yearly cost of heating and cooling this home at each NatHERS star rating.
NatHERS 2022 star bands

Reaching 7.0 stars 

Saves in heating and cooling
–a year, at today’s price
Over 25 years
–
Pays back
–

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Savings added up, year by year Heating and cooling saved
What reaching 7 stars saves, added up over 40 years.
Illustrative, and set in the upgrades’ favour. NatHERS rates a home by the heating and cooling it models for a standard household, room by room, with set thermostats and hours. The lab takes NatHERS’s own limit for each rating in the zone: the most heating and cooling a home can need and still rate that many stars. A design sits at or under its rating’s limit, and upgrades chosen to reach 7.0 stars take it to just under 7.0’s, so the gap between the two limits is the most those upgrades can save. It turns that heating and cooling into electricity through the air-conditioner’s efficiency and prices it at the rate set, rising each year as chosen, with nothing taken off for money spent now being worth more than money saved later. Households that heat and cool less than NatHERS’s standard household save less, and so do homes whose rooftop solar runs the air-conditioner by day. The supply charge is the same whatever the rating, so it isn’t counted.
The same step across WA, a year
What the same step to 7 stars saves a year in each of WA’s NatHERS climate zones.
The working
    NatHERS’s star bands for this zone
    RatingMost heating and cooling, MJ/m² a yearFor this home, kWh of electricity a yearAt this price, a year

    Every WA zone
    NatHERS climate zoneAs drawn, MJ/m²7.0 stars, MJ/m²Saved, kWh of electricitySaved a year
    Sources. Star bands: NatHERS 2022 star bands, decimal limits, for the Chenath 3.22 and 3.23 engines that NCC 2022 ratings use. Climate zones: NatHERS climate zones by postcode, September 2025, with each postcode’s principal zone first. Price: Synergy Home Plan (A1), 33.2621 c/kWh including GST from 1 July 2026, as published by Energy Policy WA. Suburbs: GeoNames, CC BY 4.0.

    02Where the numbers come from

    Published figures, shown in full.

    The lab adds nothing of its own to NatHERS’s figures. Each step uses a published source, and the working under the charts shows every number for the home on screen.

    1. NatHERS’s star bands

      For each of its 69 climate zones, NatHERS sets the most heating and cooling a home can need and still rate each tenth of a star, from 0.5 to 10. The lab uses the 2022 star bands, for the engines NCC 2022 ratings run on, as published.

      MJ/m² a year

    2. NatHERS climate zones by postcode

      A WA suburb takes its postcode’s principal NatHERS zone, from NatHERS’s September 2025 list. Where an assessor could use another zone, the lab says so, and the zone can be chosen by hand.

      Climate zone

    3. The air-conditioner’s efficiency

      The heating or cooling a system delivers for each kWh of electricity it uses: its annual coefficient of performance and energy efficiency ratio, as the GEMS energy rating register lists them for NatHERS Whole of Home.

      ACOP and AEER

    4. WA’s household rate

      Synergy’s Home Plan (A1), WA’s regulated household rate: 33.2621 c/kWh including GST from 1 July 2026, up 2.75% on the year before, as Energy Policy WA publishes it.

      c/kWh
    NatHERS 2022 star bandsNatHERS climate zones, September 2025NatHERS Technical Note, October 2024Energy Policy WA

    03The assumptions

    The best case for the upgrades.

    Where the lab has to assume, it leans the upgrades’ way, so the savings it shows sit at the generous end of what a household would see.

    1. The most the step can save

      Each rating covers a band of heating and cooling. The lab measures from the top of the as-drawn rating’s band to the top of 7.0 stars’, the most that upgrades chosen to reach 7.0 stars can save.

      Star bands

    2. NatHERS’s standard household

      NatHERS heats and cools each room to its zone’s thermostat settings and hours, for a standard household, all year. Households that heat and cool less save less.

      Use

    3. Rising prices, counted in full

      The price rises 2.75% a year, as it did on 1 July 2026, and every future dollar saved counts in full, though money saved in twenty years is worth less than money spent today.

      Price

    4. No rooftop solar

      Every kWh is bought at the household rate. Where rooftop solar runs the air-conditioner by day, a kWh saved is worth only the export payment given up, so set the price lower.

      Solar

    04Questions

    Straight answers on the last star.

    Is this my power bill?

    No. It prices the heating and cooling NatHERS models for a standard household, which is how a rating is set, not a forecast of what any household will use. A real bill depends on how the home is lived in, its appliances, any solar, and the supply charge.

    Why is the supply charge left out?

    It’s the same whatever the home’s rating, so reaching 7 stars doesn’t change it. Only the electricity used for heating and cooling does.

    Does a Performance Solution mean a less efficient home?

    No. Under H6V2, the design is modelled beside a reference house built to the Code’s Deemed-to-Satisfy Provisions, and must need no more heating and no more cooling than it. That meets the same performance requirement, H6P1, by a different test. A Performance Solution isn’t given a star rating, and we don’t claim one for it. More in ‘Is a Performance Solution a loophole?’

    Why does the north save more?

    NatHERS’s bands are far wider where air-conditioning runs much of the year. For the same 200 m² home going from 6.0 to 7.0 stars, the step saves two and a half to four times as much in Broome, Port Hedland, Halls Creek and Wyndham as on Perth’s coast, and upgrades pay back sooner. The lab shows every WA zone side by side.

    Where do I find my design’s rating as drawn?

    From the assessor’s first run, before any upgrades are added. If the design hasn’t been rated, send us the plans: the Pathway Check rates it and compares the routes.

    What if the house has rooftop solar?

    Then some of its heating and cooling runs on its own panels, and a kWh saved by day is worth only the export payment given up, not the household rate. Set the electricity price lower to see the effect: the savings fall with it.

    Pathway Check

    Which route suits your design?

    Send your plans and we’ll compare the routes, including when the answer is NatHERS.

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