How the GHG Inventory Is Calculated: Steps, Methods, Emission Factors

Once the boundaries are set, the work of a corporate carbon footprint is arithmetic with rules attached. Section 6 of the TGO Requirements for Calculating and Reporting the Carbon Footprint of an Organization (2022) describes the calculation as a sequence of five steps, and sections 8 and the appendices add the rules on factors, GWP, base year and uncertainty. This article follows that order.

#The five steps

The requirements state that calculating an organization's emissions and removals consists of: (1) identifying GHG sources and sinks; (2) selecting the calculation method; (3) selecting or developing emission factors; (4) calculating emissions and removals; and (5) establishing the base-year inventory (TGO CFO Requirements, 2022, p.21).

#Step 1: identify sources and sinks

The organization must identify and document all relevant sources and sinks within the reporting boundary, activity by activity, following the category structure of section 5.2.5. It may exclude sources and sinks that do not significantly affect the total, but must explain the reasons (TGO CFO Requirements, 2022, p.21). The same allowance, with the same obligation to explain, is repeated in the calculation step itself (TGO CFO Requirements, 2022, p.23). In practice this means an exclusion list with reasons is part of the inventory, not an afterthought.

#Step 2: select the calculation method

The method must reduce uncertainty and give results that are accurate, consistent and reproducible, and the choice should weigh technical suitability against cost (TGO CFO Requirements, 2022, p.22). The organization must document the characteristics of each type of data it uses, and the requirements note that data may be primary (including site-specific data) or secondary; the example given is the average fuel consumption of trucks together with the standard characteristics used to calculate it (TGO CFO Requirements, 2022, p.22).

Where there is no direct measurement, the organization must select or develop a calculation model and explain in writing why it chose it, considering seven properties: accuracy, limitations, uncertainty and conditions of use, reproducibility, acceptance, origin and recognition, and consistency with the intended users (TGO CFO Requirements, 2022, p.22). The document then gives the formula that most models reduce to: GHG quantity equals activity data multiplied by the emission or removal factor (TGO CFO Requirements, 2022, p.23).

#Step 3: select or develop emission factors

An emission factor used with activity data must meet five conditions: it comes from a recognised source; it fits the specific source or sink; it is current at the time of calculation; it takes uncertainty into account so the result is correct; and it is consistent with the intended application of the inventory (TGO CFO Requirements, 2022, p.23). The organization must explain the source of each factor and why it is appropriate, and, importantly, if a factor is changed from the one used before, the base-year inventory must be recalculated (TGO CFO Requirements, 2022, p.23).

When primary factors cannot be collected, secondary factors from credible published sources may be used, ranked by priority as set out in Appendix 9 (TGO CFO Requirements, 2022, p.23). That appendix ranks sources in five tiers: first, databases published by domestic organizations directly involved in the activity; second, the Thai national life-cycle inventory database (Thai LCI Database) maintained by TIIS; third, peer-reviewed Thai theses and research; fourth, generally published databases such as LCA software, industry-specific or country-specific databases; and fifth, data published by international bodies such as the IPCC and UN organizations (TGO CFO Requirements, 2022, p.77). The order is deliberate: the closer the factor is to the actual Thai activity, the higher it ranks.

#Step 4: calculate and convert with GWP

The organization must calculate according to the method chosen in step 2, report for the defined period, and convert each gas into tonnes of carbon dioxide equivalent using 100-year GWP values as specified by TGO (TGO CFO Requirements, 2022, p.23). It must calculate as completely as it can within the organizational boundary and record the work in writing (TGO CFO Requirements, 2022, p.23).

Appendix 1 lists the gases and their 100-year GWP values from the IPCC Fifth Assessment Report (AR5). Carbon dioxide is 1, methane 28, fossil methane 30 and nitrous oxide 265; among hydrofluorocarbons, HFC-32 is 677, HFC-134a is 1,300 and HFC-23 is 12,400 (TGO CFO Requirements, 2022, p.36). These are the values a Thai inventory should use unless TGO states otherwise, and they are the reason a small refrigerant leak can outweigh a large fuel bill.

#Step 5: the base year

The organization must set a base year for comparison, or to meet the rules of the registration and certification body, or for other uses of the inventory. The base year can be a specific period, such as a year or a seasonal part of a year, or an average of several periods. If there is not enough historical information, the first year for which emissions are calculated may serve as the base year (TGO CFO Requirements, 2022, p.24). In setting it, the organization must quantify base-year emissions with data representative of the current reporting boundary, choose a year whose data can be verified, explain the choice, and develop the base-year inventory in line with the requirements. The base year may be changed, but the reason must be explained (TGO CFO Requirements, 2022, p.24).

Section 6.6.2 then requires a documented procedure for reviewing and recalculating the base year so it stays representative. Recalculation is triggered by a change in reporting structure or organizational boundary, such as a merger, acquisition or divestment; a change in calculation method or emission factors; or the discovery of an error, or cumulative errors, that are significant. It is not required for changes in production volume or for opening or closing facilities. Any recalculated base-year inventory must be documented (TGO CFO Requirements, 2022, p.24).

#Managing quality and assessing uncertainty

Section 8 describes a quality management approach in six parts: form a team with defined responsibilities and training; develop a quality management plan covering data collection at every level of the organization; check the quality of the calculation from the assessment principles and boundaries through to equipment, activities, factors and methods; identify and review materiality; have the inventory verified by internal and external experts; and review errors to define corrective actions (TGO CFO Requirements, 2022, p.29).

On uncertainty, the organization must assess the uncertainty associated with its quantification approach, including the data and models used, and evaluate the uncertainty of the inventory. Where a quantitative estimate is not possible or not cost-effective, it must explain why and make a qualitative assessment instead, and it may draw on the methodology in Appendix 11 (TGO CFO Requirements, 2022, p.29).

Appendix 11 gives a worked example for a company with three sources: fuel oil measured continuously with a supplier factor, electricity from invoices with a Thai database factor, and petrol for staff travel estimated with an IPCC factor (TGO CFO Requirements, 2022, p.79). Data collection is scored 6 for continuous automated collection, 3 for meters and invoices, and 1 for estimates; emission factors are scored 4 for measured factors, 3 for manufacturer or national factors, 2 for regional factors and 1 for international factors. Combined scores are then graded: 1 to 6 means high uncertainty and poor data quality, 7 to 12 slight uncertainty and moderate quality, 13 to 18 low uncertainty and good quality, and 19 to 24 low uncertainty and excellent quality (TGO CFO Requirements, 2022, p.79). The scheme is simple, but it makes an important point explicit: a measured activity value paired with an international default factor is only medium quality.

#How CarbonBiz reflects these rules

The steps above map directly onto how the App stores data. Each activity records the method string and whether the value was measured, calculated or estimated, which is what the Appendix 11 scoring needs. Emission factors carry their source, standard version and GWP version, system factors are locked, and custom Tier 2 and Tier 3 factors must state a reason and source URL. When a factor is updated, the previous version is kept and every dependent activity can be recalculated, which is the mechanism the base-year recalculation rule requires. Goals hold the base-year value, and Fr-04.2 reproduces the base-year account in the same format as the reporting year.