Can an NGER estimation method be changed each year?

No. Under the NGER (Measurement) Determination 2008, once an NGER estimation method is chosen for a source it must be used for at least 4 consecutive reporting years. A reporter can move to a higher method at any time, but must then keep it for at least 4 years before moving to a lower method.

Changing NGER Estimation Methods: The Four-Year Rule

An NGER estimation method cannot be changed freely each year. Once a method is chosen for a separate occurrence of a source, the reporter must continue to use it for at least four consecutive reporting years, under Division 1.2.2 of the NGER Measurement Determination.

Key points

The same method must be used for at least four consecutive reporting years
A higher method can be adopted at any time
A higher method must be kept four years before moving lower
Criteria AA and AAA lock in for later years
Method 1 is not available for some large generating units

Moving to a higher method

A reporter can switch to a higher method, such as from Method 1 to Method 2, at any time. It must then use the higher method for at least four years before changing to a lower method. The Determination also allows a lower method where an audit has found the method in use does not comply.

Measurement criteria also lock in

For solid, gaseous and liquid fuels, a facility that uses criterion AA in a year can use only criterion AA in later years. A facility that measures at the point of combustion or consumption under criterion AAA must keep that measurement in later years.

Where Method 1 is not available

The Determination removes Method 1 for some sources. It cannot be used for carbon dioxide from the main fuel at a generating unit of 30 megawatts or more that generates more than 50,000 megawatt hours in the year. Open-cut coal mines covered by the Safeguard Mechanism are being moved to Method 2 or 3 for fugitive methane.

Why higher methods are used

Higher methods replace national average factors with the properties of the fuels and materials a facility actually uses, so the reported figure reflects its own coal, gas or diesel. The Regulator has run a thematic audit on the use of higher methods for open cut coal mining. A reporter that moves a source from Method 1 to Method 2 must then keep Method 2 or higher for at least four reporting years before it can return to Method 1.

Planning a change

Because a method choice binds for four years, it is best made once the data, sampling and metering it needs are in place. The Determination is amended every year, so the methods available for each source should be checked against the version in force for the reporting year.

References & sources

Primary regulatory documents and authoritative references underpinning this answer.