Start with boundaries, not spreadsheets
The most common mistake is to begin collecting data before deciding what belongs in the inventory. The first step is to define the organisational boundary (which entities, sites and operations are included), the operational boundary (which emissions sources are direct, energy-related or indirect), and the reporting period.
For most companies this means choosing between operational control and financial control for the organisational boundary, and between equity share and control for joint operations. Getting this wrong means restating the inventory later.
Design the data collection before collecting it
Scope 1 and 2 data usually come from utility invoices, fuel records, refrigerant logs and fleet-management systems. Scope 3 data sits in procurement systems, travel booking tools, expense records and supplier correspondence.
Create a data-collection template that records the activity data, the source document, the person responsible, the emission factor used, and any assumptions or estimates. This is the foundation of traceability and the first thing an external verifier will ask for.
Choose the right emission factors
Emission factors translate activity data into tonnes of CO2e. For Scope 1 and 2 this is usually straightforward: national grid factors for electricity, DEFRA or equivalent for stationary combustion and transport.
Scope 3 is more complex. Spend-based factors are a reasonable starting point for screening, but activity-based and supplier-specific factors produce better data where the information exists. The right approach depends on the quality of your data and the materiality of each category.
Document everything as you go
A GHG inventory is only as good as its documentation. Record the methodology, boundaries, factors, assumptions, estimates, and changes from the previous year. This is not just good practice; it is what makes assurance possible and what protects the organisation when the numbers are questioned.