How home energy ratings are calculated
This page provides details of what is measured in a home energy rating, and how ratings are calculated. The thermal performance of the building structure as well as the performance of the water heating and space heating systems are rated, taking into account factors like the climate in your area.
Rating the home, not the occupants
A home energy rating is based on the performance of the home itself, rather than the behaviour of its occupants. People choose to use energy in their homes in different ways - for example, how long they spend in the shower, or how long they heat their homes for. Standard assumptions in terms of occupancy and indoor temperatures are used in a rating, so that every home can be assessed on the same basis. This also means that ratings are consistent and comparable across New Zealand.
Energy rating tool - AccuRate NZ
Assessors use a computer modelling tool called AccuRate NZ to generate accurate and comparative ratings across New Zealand. AccuRate NZ is a sophisticated tool that allows the assessor to model the energy efficiency performance of a home. It's even capable of handling very complex homes.
AccuRate was originally developed by Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) and has been used in Australia for some time.
AccuRate NZ has been adapted for use in New Zealand conditions by changing variables such as climate data and construction materials.
What is actually measured?
There are three components measured in a home energy rating:
- The building itself (design, construction and orientation of the home)
- The water heating system
- The room heating system.
The building
The thermal performance of the home design is measured. This has the biggest impact on a home's overall energy efficiency. Thermal performance includes:
- The design of the building
- How a building is oriented to capture the sun's energy
- Building materials and how they are used (including their thermal resistance and their ability to absorb and retain heat)
- Window size, type and orientation
- Shading - this covers the size of the eaves and shading from nearby structures (buildings, pergolas, roll-down blinds, sliding shutters, etc)
- Ventilation - a calculation of the airflow through the house.
AccuRate will then determine how much heating and cooling is needed throughout the year to keep the home at a comfortable and healthy indoor temperature.
The water heating system
The overall efficiency of the water heating system is assessed.
The room heating system
The room heating rating is a measure of how well and how efficiently the home's heating need is met by the installed room heating appliances.
What assumptions are made?
AccuRate is based on standard assumptions for climate conditions, indoor temperatures and the number of home occupants. This is so every home is assessed on the same basis. It also means ratings are consistent and comparable across the country.
AccuRate divides New Zealand into 18 separate climate zones, using data from the National Institute of Water and Atmospheric Research (NIWA).
Data variables include temperature, humidity, wind and solar irradiance.
When a house is assessed, the model will assume that the house is exposed to the climate data set for that zone. This is designed to reflect regional climatic variations across New Zealand.
For example, if you had exactly the same home in Invercargill and in Kaitaia (including solar orientation), the Invercargill home would need more heating to maintain the same healthy and comfortable indoor temperature.
AccuRate also includes assumptions about the:
- Number of people living in the home (which is based on the number of bedrooms)
- Hot water use (which is based on the assumed number of people living in the home)
- Heating patterns (what temperature range is maintained in the home).
To find out more about assessors, including how to become an assessor, see Home Energy Rating Assessors on the ABSA website.
