Step 5: How is the calculation done?

The core formula is:

Emissions = activity data x emission factor x GWP, added up across sources and gases.

Illustrative example: a small forging unit

A unit uses 10,000 litres of diesel in its DG set and 500 MWh of grid electricity in a year.

Diesel (Scope 1), using IPCC 2006 Guidelines, Volume 2:

• Mass: 10,000 L x 0.84 kg/L = 8,400 kg = 0.0084 Gg. (0.84 kg/L is an assumed density; use the figure on your supplier's specification.)
• Energy: 0.0084 Gg x 43.0 TJ/Gg (default net calorific value, Table 1.2) = 0.3612 TJ.
• CO2: 0.3612 TJ x 74,100 kg/TJ (default, Table 2.3, manufacturing industries) = 26,765 kg.
• CH4 and N2O: 0.3612 TJ x 3 kg/TJ x 29.8 = 32 kg CO2e, and 0.3612 TJ x 0.6 kg/TJ x 273 = 59 kg CO2e (Table 2.3 defaults, IPCC AR6 GWPs).
• Total: about 26.9 tCO2e.

Electricity (Scope 2, location-based):

• 500 MWh x 0.675 tCO2/MWh = 337.5 tCO2, using the weighted-average grid factor for FY 2025-26 from the CEA CO2 Baseline Database, Version 22.0 (August 2026). In a real inventory, use the CEA version and financial year that match your reporting year.

Result: about 364.4 tCO2e for Scope 1 and 2. If the unit made 2,000 tonnes of forgings, its intensity is about 0.18 tCO2e per tonne. In this example electricity is more than 90% of the total, which shows where to look first.

A spreadsheet is fine for this. Keep every factor, its source, version and year in a separate tab.