Every emissions calculation depends on an emission factor: a number that converts a physical activity (litres of fuel, kWh of electricity) into tonnes of CO₂-equivalent. In Australia, the primary source for these factors is the National Greenhouse Accounts (NGA) Factors, published annually by DCCEEW.

Because the NGA Factors are updated every year — and because methodology and GWP values can also change — the version of the factors you use matters. Using last year’s grid factor for this year’s electricity calculation isn’t technically wrong if they’re identical, but you can’t know they’re identical unless you check. And for an assurance provider, “I assumed it hadn’t changed” is not an acceptable answer.

Why factor versioning matters

Factors change. The electricity grid emission factors change every year as the generation mix evolves. A state that added significant solar capacity in a given year will have a lower emission factor for that year. Using an older factor overstates your Scope 2 emissions; using a newer factor for a prior period may make comparisons inconsistent.

GWP values can change. The NGA Factors use specific global warming potential (GWP) values for non-CO₂ gases (methane, nitrous oxide, refrigerants). These are periodically updated by the IPCC and adopted in the NGA. A change in the methane GWP from AR4 (25) to AR5 (28) affects any calculation involving CH₄.

Methodology can change. The method for calculating electricity emissions for an isolated grid may differ from prior years. New fuel categories may be added. Default factors for estimation may be revised.

Assurance requires traceability. An assurance provider reviewing your Scope 1 and 2 figures will ask: “Which version of the NGA Factors did you use?” If you can’t answer with precision, the calculation is not traceable.

What to record for each factor used

For every emission factor applied in your calculations, record:

Factor source: “NGA Factors” (not just “DCCEEW” or “government factors”)
Publication year/version: “NGA Factors FY2025” (the year the factors apply to, not the publication date)
Specific table and row: “Table 3 — Grid Emission Factors — Victoria (NEM)” or “Table 4 — Fuel Combustion — Diesel, mobile”
Factor value: The actual number (e.g., 0.94 kg CO₂e/kWh)
Date accessed: When you referenced the document

This creates a complete reference that allows anyone to locate the exact source and verify the factor used.

Factor versioning by reporting period

The standard practice is to use the NGA Factors that correspond to the reporting period:

For a reporting period ending 30 June 2025 (FY2025), use NGA Factors FY2025
For a reporting period ending 31 December 2025, typically use the most recently published NGA Factors at the time of calculation

The NGA Factors are typically published in the second half of the year following the reference year — meaning the FY2025 factors may not be available until late 2025 or early 2026. If you’re calculating FY2025 emissions before the FY2025 factors are published, you may need to use FY2024 factors as a provisional calculation and update when the new version is available.

Document this situation explicitly: “Preliminary calculation using NGA FY2024 factors; to be updated when FY2025 factors are published.”

Managing prior period comparatives

AASB S2 will require prior period comparative figures once reporting matures (from year two onwards for most groups). When restating prior year figures, you have two options:

Option A: Restate using current-year factors. This produces internally consistent comparatives but requires you to recalculate prior periods. It’s more work but presents a cleaner picture.

Option B: Retain original factors for prior periods. This preserves the calculation as it was done at the time but may make year-on-year comparisons misleading if factors changed materially.

Document which approach you’ve used, and apply it consistently. Switching between approaches without disclosure undermines year-on-year comparability.

Practical versioning in spreadsheets vs systems

In spreadsheets: Create a dedicated “Factors” tab that lists every factor used, with its source reference, value, and the reporting period it applies to. All calculation tabs should reference this tab rather than hard-coding factor values. This way, updating a factor updates all calculations that use it.

In a purpose-built system: Factor management should be built in — factors are stored with their source, version, and applicability dates, and calculations are linked to the factor version used at the time of calculation. This creates immutable records that can be reviewed without reconstructing the spreadsheet state.

What happens when factors change after you’ve reported

If DCCEEW significantly revises a prior year’s factors (unusual but not impossible), you may need to consider whether to restate prior period figures. The AASB S2 guidance on prior period errors applies — material errors should be corrected and disclosed.

For routine annual factor updates (the normal case), no restatement of prior periods is required. The change is accounted for going forward.

Ayika stores every calculation with its factor reference and version, so your emissions figures are always traceable to the exact source data used. See how factor management works.

How to Version Emission Factors and Calculations