An A-rated house has a Building Energy Rating in the highest band, from A1 down to A3, indicating very low energy use and carbon emissions relative to its size, usually achieved through excellent insulation, an efficient heating system such as a heat pump, and often on-site renewables. Most new homes built to current NZEB standards fall into this band, while achieving it in an older home usually requires substantial retrofit works.
The Building Energy Rating scale runs from A1, the most efficient, down to G, the least efficient, based on the calculated primary energy use and carbon dioxide emissions of a dwelling per square metre per year. The A band is further divided into A1, A2 and A3, each representing progressively lower (but still very good) energy performance, and a home in any of these sub-bands is generally described as 'A-rated.'
A-rated homes typically combine a highly insulated and airtight building fabric, high-performance windows and doors, an efficient heating system, most commonly a heat pump, and often a renewable energy contribution such as solar panels. Because of these requirements, most new houses built to current Nearly Zero Energy Building standards achieve an A rating, usually A2, as this is broadly what current building regulations are designed to deliver.
For existing, older housing stock, reaching an A rating usually requires a comprehensive 'deep retrofit,' combining wall and attic insulation upgrades, window and door replacement, a heat pump installation, and often solar PV, since simply changing one element such as the boiler is rarely enough on its own to move an older home into the A band.