GHG Emissions Inventory: Method

GHG Emissions Inventory

A greenhouse gas inventory adds up what an organization emitted in a year. Each line of it is a measured or billed quantity (the activity data: therms of gas, gallons of fuel, kilowatt hours, kilograms of refrigerant topped up) multiplied by an emission factor. This tool carries the factor tables of the EPA GHG Emission Factors Hub of 15 January 2025 cell for cell, follows the calculation steps of the EPA Center for Corporate Climate Leadership guidance documents, and reports the result in the scopes of the GHG Protocol.

Scopes

Scope 1 is what the organization emits itself: fuel burned in its boilers, furnaces and generators (stationary combustion), fuel burned in its vehicles and equipment (mobile combustion), and refrigerants and other gases that escape from its equipment. Scope 2 is the emission at the power plant or district plant for the electricity, steam, heat or cooling it buys. Scope 3 is everything else in the value chain; the tool covers the categories for which the hub prints factors: transportation and distribution by distance (categories 4 and 9), waste generated in operations (5), business travel (6) and employee commuting (7). The scope 3 total is therefore the total of the entered categories, not a complete scope 3 inventory.

One row

For every gas, emissions = activity × emission factor. The three combustion gases are carbon dioxide, methane and nitrous oxide, and the row total is expressed as carbon dioxide equivalent,

CO₂e = CO₂ + GWP_CH4 · CH₄ + GWP_N2O · N₂O + Σ GWP_i · m_i

where the last term is the mass of each other gas released times its global warming potential. The first default row is the worked example of the EPA stationary combustion guidance: 61,500 therms of natural gas are 6,150 mmBtu, times 53.06 kg CO₂, 1.0 g CH₄ and 0.10 g N₂O per mmBtu gives 326.3 t CO₂, 6.15 kg CH₄ and 0.615 kg N₂O, and with GWPs of 28 and 265 the row is 326.7 metric tons CO₂e, as the guidance prints.

Stationary combustion

Hub Table 1 (from 40 CFR Part 98, Tables C-1 and C-2) lists 63 fuels with a default higher heating value and two sets of factors: per mmBtu, and per unit of fuel (short ton of a solid, standard cubic foot of a gas, gallon of a liquid). A quantity entered in an energy unit takes the per-mmBtu factors (the guidance's Equation 2, the preferred form when the bill states energy); a quantity in a mass or volume unit takes the per-unit factors (Equation 1). The per-unit columns are the per-mmBtu factors times the default heating value, rounded as printed, so the two routes can differ in the third or fourth figure. The kraft pulping liquors have per-mmBtu factors only.

Unit conversions use the exact definitions of NIST SP 811: 1 Btu_IT = 1055.05585262 J (so 1 mmBtu = 1.05505585 GJ = 293.071 kWh), 1 lb = 0.45359237 kg, 1 short ton = 907.18474 kg, 1 gal = 3.785411784 L, 1 ft³ = 0.028316846592 m³, 1 barrel = 42 gal, 1 mile = 1.609344 km. The therm is 100,000 Btu, 0.1 mmBtu, as in the EPA guidance. Gas volumes are converted as volumes; a cubic metre at other reference conditions than the standard cubic foot's should be corrected before it is entered.

Biomass. The CO₂ from burning a biomass fuel (wood, agricultural residues, landfill gas, biodiesel, ethanol, pulping liquor) is reported as biogenic CO₂, separately and outside the scopes; its CH₄ and N₂O count in scope 1. The tool follows that rule for the stationary fuels of the biomass groups of Table 1 and for biodiesel and ethanol in vehicles.

Mobile combustion

CO₂ comes from the fuel: gallons (or standard cubic feet of CNG) times the factor of hub Table 2. CH₄ and N₂O depend on the engine and its emission controls rather than on the fuel alone. For on-road vehicles Tables 3 and 4 give grams per vehicle-mile by vehicle type, fuel and model year, so those two gases come from the distance; for non-road equipment (ships, locomotives, aircraft, construction, agricultural and other equipment) Table 5 gives grams per gallon, so they come from the fuel. A row with fuel but no distance reports CO₂ only, and says so. A model year after the last one listed (2022) takes the last row. Methanol vehicles are left out because Table 2 has no CO₂ factor for methanol; electric vehicles belong in the electricity table.

Refrigerants and other gases

The row is the mass released in the year times the gas's GWP. The mass is what was added to the equipment to bring it back to full charge, or the charge times an annual leak rate (the EPA guidance's screening method: capacity × leak rate × fraction of the year in use), entered as a mass. Hub Table 11 lists 32 gases and Table 12 thirty blends; a blend's GWP counts only its HFC and PFC constituents, so blends of HCFCs alone show zero, as in the table. Any other gas can be entered with its own GWP.

Global warming potentials

A GWP is the warming of a kilogram of a gas over 100 years relative to a kilogram of CO₂. The values have been revised in each IPCC assessment, and an inventory must state which set it uses and keep to it across the years it compares.

Set	CH4	N2O	HFC-134a	SF6	Source of the values in the tool
AR4 (2007)	25	298	1,430	22,800	EPA hub of 2023, Table 11; 40 CFR Part 98 Table A-1 (2024 edition)
AR5 (2013)	28	265	1,300	23,500	EPA hub of 2025, Table 11
AR6 (2021)	29.8 / 27.0	273	1,530	24,300	IPCC AR6 Working Group I, Tables 7.15 and 7.SM.7

AR6 gives methane two values: 29.8 for methane of fossil origin and 27.0 for non-fossil methane. The tool applies 27.0 to the methane of biomass fuels and biofuels and 29.8 to all other methane. Under AR4 and AR6 a blend's GWP is the composition-weighted sum of its constituents' values; under AR5 it is the value printed in Table 12. The Entered choice replaces the CH₄ and N₂O values and leaves the other gases at AR5. The waste factors of Table 9 and the entered electricity, steam and market factors are already in CO₂e and are not re-weighted.

Purchased electricity: two totals

Location-based: megawatt hours times the average emission rate of the grid region. For the United States those are the eGRID subregion total output rates in lb per MWh for each gas (1 lb = 0.45359237 kg). The hub's Table 6 prints eGRID2023 as first released in January 2025; EPA revised the eGRID2023 summary tables afterwards (revision 2, June 2025), and several subregions changed by a few percent. The tool defaults to revision 2 and offers the hub's printing as the other choice. The non-baseload rates in the same table are for estimating the effect of a change in consumption, not for an inventory, and are not used. For a site elsewhere choose Entered factor and give the grid's rate in kg CO₂e per MWh.

Market-based: the same megawatt hours priced by what the organization has contracted for. The share covered by zero-emission instruments (renewable energy certificates, a power purchase agreement, a utility green tariff) is taken out, and the rest is multiplied by the supplier or contract rate if one is entered, otherwise by the grid rate, since no residual-mix rates are published for the United States:

market-based = MWh · (1 − instruments) · (supplier rate, or the grid rate)

Both totals are reported side by side, as the GHG Protocol's scope 2 guidance requires of an organization with any contractual instruments. Purchased steam and heat take hub Table 7 (natural gas burned at 80 percent efficiency) or a supplier factor.

Scope 3 rows

Tables 8 and 10 give factors per vehicle-mile, passenger-mile or short ton-mile; the kilometre basis converts with 1 mile = 1.609344 km and 1 short ton-mile = 1.459972 tonne-km. Table 9 gives metric tons CO₂e per short ton of each of 61 materials for each treatment (recycled, landfilled, combusted, composted, anaerobically digested); the table prints NA where a treatment does not apply to a material, and the tool reports such a row instead of inventing a factor.

Intensity, base year and target

Intensity is a total divided by a measure of activity: revenue, employees, floor area, or units of output. The base-year table shows the change, the percent change, and the compound annual rate (E / E₀)^1/n − 1 over the n years between the two inventories. The target check draws a straight line from the base-year emissions to the target-year emissions E₀(1 − cut) and compares this year's scope 1 + 2 with the point on the line. A base year is only comparable when it was calculated with the same boundaries, the same GWP set and consistent factors; when the organization or the method changes materially, the base year is recalculated.

Reduction scenarios

The what-if table reruns the whole inventory with changed activity data. Efficiency scales the stationary fuel rows, the mobile rows (fuel and distance) and the electricity rows by the entered cuts. Fuel switching removes a share of one stationary fuel's energy and supplies it with another fuel or with electricity: new energy = displaced energy × energy ratio, where the ratio is the old equipment's efficiency over the new one's efficiency or COP (0.8 / 3 ≈ 0.27 for a boiler replaced by a heat pump, 1 for a like-for-like fuel change). Electricity added by a switch is bought at the terms of the first electricity row. Renewable electricity raises the contractual-instrument share of every electricity row to the entered percentage, which lowers the market-based total and leaves the location-based total where the grid puts it. The combined line applies the three in that order, so its saving is less than the sum of the separate savings.

Uncertainty

If a row's activity is known to ± u percent, its emissions are known to ± u percent, a half-range h = u E. For a total of rows whose errors are independent the half-ranges add in quadrature, H = √(Σ h²); if every row erred in the same direction they would add directly, H = Σ h. The tool prints both. Only the activity data is ranged. Emission factors carry their own uncertainty (small for the CO₂ of a commercial fuel, large for CH₄ and N₂O factors, waste and travel factors) and GWPs theirs; neither is included.

What the tool does not do

It does not set the organizational boundary (equity share or control), does not cover process emissions, fugitive methane from gas systems, or the scope 3 categories without a hub factor (purchased goods and services, capital goods, fuel- and energy-related activities including transmission losses, leased assets, use of sold products, investments), and does not verify data. The factors are those of the United States; for sites elsewhere the fuel factors are a reasonable first estimate and the electricity factor must be entered.