NatHERS vs. HERS: How Australian and US Home Energy Ratings Compare

If you've bought, built or renovated in Australia you've probably seen a NatHERS star rating; in the US, the equivalent conversation is usually about the HERS Index or an ENERGY STAR certification. They're answering a similar underlying question — how much energy will this home's design need? — but they're built differently and score in opposite directions, which trips people up when comparing across the two systems.

NatHERS: 0 to 10 stars, higher is better

The Nationwide House Energy Rating Scheme models a home's design (orientation, insulation, glazing, ventilation, thermal mass and climate zone) to estimate its heating and cooling energy demand, then expresses that as a star rating from 0 to 10 — a low-rated design needs a lot of energy to stay comfortable; a high-rated one needs very little. Most Australian states require a minimum NatHERS rating (commonly 7 stars in the current National Construction Code, though requirements and transition timing vary by state) for new homes and major renovations. NatHERS assesses the building envelope and passive design — it does not directly account for solar panels, batteries or appliance choice.

HERS Index: a score where lower is better

The US Home Energy Rating System Index compares a home's modelled energy use against a reference home of the same size and shape built to a fixed 2006 energy-code baseline, which is assigned a HERS Index of 100. A home needing half the energy of that reference home scores 50; a net-zero-energy home scores 0 (or can go negative if it produces more energy than it uses). Unlike NatHERS, HERS Index assessments commonly do account for on-site generation like solar, which is why a HERS score isn't purely a measure of the building envelope the way a NatHERS star rating is.

ENERGY STAR certification vs. HERS Index

ENERGY STAR certified new homes is a related but separate US program — it sets a required HERS Index threshold (among other requirements) that a home must meet to earn the certification, rather than being a numeric scale itself. Think of HERS Index as the underlying measurement, and ENERGY STAR as one certification that uses it as a bar to clear.

Why this matters for electrification

Both systems exist because the building envelope sets the floor for how much heating and cooling load any system — including a heat pump — actually has to handle. A well-insulated, well-glazed, high-rated home lets you install a smaller, cheaper heat pump and still stay comfortable; a poorly-rated one forces oversizing and higher running costs regardless of how efficient the equipment itself is. See our Green Building and Heat Pumps pillars for how insulation and glazing choices translate into real sizing decisions.