How Home Energy Ratings are calculated
This page provides details of what is measured in a Home Energy Rating assessment and how ratings are calculated.
On this page:
- Rating the home, not the occupants
- Energy rating tool: AccuRate
- What is measured – required data
- What assumptions are made?
Rating the home, not the occupants
A Home Energy Rating is based on the performance of the home itself, rather than the behaviours of its present or future occupants. This is the most common approach to energy ratings internationally. There are endless variations on how people choose to use energy in their homes (how high they like to turn up their heating, how long they spend in the shower), and how different owners will use energy over the life of the home. Standardisations are used so that every home can be assessed on the same basis and that ratings are consistent and comparable across New Zealand.
Energy rating tool – AccuRate NZ
To generate accurate and comparative ratings across New Zealand, assessors use a computer modelling tool called AccuRate NZ. This is a sophisticated tool that allows the assessor to model the energy efficiency performance of a home over a range of different spaces and zones throughout the home, so it is capable of handling even the most 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 changes to variables such as climate data and construction materials.
What is actually measured?
There are three components measured in a Home Energy Rating assessment:
- the building (design, construction and orientation of the home);
- the water heating system; and
- the room heating system.
The following provides a summary of each component:
1. The building
This is a measure of the thermal performance of the house design, and has the biggest impact on a home’s overall energy efficiency. It includes:
- the design of the building;
- how a building is oriented – that is, its potential for solar gain;
- building materials and how they are employed (including the degree of thermal conductivity, conductance and resistance);
- window size, type and orientation;
- shading – covering 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; and
- thermal mass of the materials used (the ability of a given material to hold and absorb heat and distribute it throughout the evening).
The tool will then determine how much heating and cooling is needed throughout the year to keep the home at a comfortable and healthy indoor temperature.
2. The water heating system
This is a measure of the overall efficiency of the water heating system (e.g. appliance efficiency etc).
It tells you how efficient a home's water heating system is and how well it is sized to meet the home's needs.
The water heating system is an important contributor to a home’s overall energy efficiency as water heating accounts for around a third of the average New Zealand home’s energy use.
3. The room heating system
The room heating rating is linked to the building rating. The building rating determines what amount of heating a home needs, and the room heating rating looks at how well this need is met (and how efficiently).
It includes:
- an assessment of the efficiency of the main heating appliances; and
- an assessment of the heating requirements of each of the rooms.
What assumptions are made?
To ensure that every home will be assessed on the same basis and that ratings are consistent and comparable across the country, the energy rating tool uses two key standardised assumptions: climatic data and standard occupancy settings.
Climatic data
The energy rating tool divides New Zealand into 16 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.
To ensure consistency and enable comparison between energy ratings of different homes, standard assumptions are made for the home. These assumptions include:
- 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); and
- heating patterns (what temperature range is maintained in the home).