SIGNAL: Average CO2

Average emissions factors, including WattTime’s Average Operating Emissions Rate (AOER), describe the emissions footprint of electricity consumption in a grid region for the purpose of attributional accounting, or allocating emissions to electricity consumption.

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What is it?

The Average Operating Emissions Rate (AOER) is a location-specific, consumption-based average emissions rate that includes electricity production within a particular grid region’s operational boundary, the proportional fuel mix of electricity imported from each neighboring electricity grid, and energy storage effects. The units of AOER are the amount of CO2 and CO2 equivalent directly emitted from combustion allocated to a unit of energy consumed (CO2e lbs/MWh).

How is it used?

For accounting only. The AOER is provided specifically for use in GHG Protocol Scope 2 reporting, to determine the total annual emissions footprint of the electricity consumption of a company’s assets. The AOER allows this accounting to be done using more recent and frequent emissions factors. For this purpose, it is considered the highest quality as defined by GHG Protocol Scope 2 Guidance (2015), and the highest quality in most cases as defined by the GHG Protocol’s Scope 2 Proposal released in October 2025.

But caution is warranted, since load-shifting or renewable-procurement decisions based on this signal won't necessarily reduce emissions. This is because, as the GHG Protocol says, measuring a company’s carbon footprint with Scope 2 “does not quantify impacts of an organization’s individual actions.” Learn more about why marginal emissions are better than average emissions for impact accounting.

Temporal coverage

Granularity: Hourly
Historical: At least 5 years in most regions
Recency/Latency: within 72 hours
Forecast: None

Geographic coverage

Granularity: Balancing region level (as small as a single utility, often as large as a country)
Coverage: Select AOER on the coverage map here

FAQ

Are AOER data “consumption-based” or “generation-based” factors? WattTime’s AOER, as of model date 2026-04-01 is a consumption-based factor, designed for use with GHG Protocol Scope 2 location-based accounting. This means that it accounts for import/export between regions, as indicated in the Scope 2 guidance.

Does AOER include the effect of transmission & distribution losses on emissions? No. Since it is provided for use with GHG Protocol Scope 2, it specifically excludes the effect of T&D losses. Scope 2 Guidance says that for the inventory of an energy consumer, the emissions attributed to T&D fall under Scope 3. See “Transmission and Distribution Losses” at the bottom of this page for more resources on this topic.

Is AOER measured in pounds of CO2 or pounds of CO2e (equivalent)? CO2e. AOER units are the amount of equivalent CO2 per unit of energy (CO2e lbs/MWh). As of model date 2026-04-01, it incorporates an estimate of the (relatively small) contribution of combustion-related emissions of other greenhouse gases, including CH4 and N2O.

Are any other lifecycle emissions included? AOER includes operational emissions only. These are the direct CO2 emissions from combustion for electricity generation. Any other upstream or downstream emissions are excluded (e.g., from fuel extraction and transportation).

How is energy storage accounted for in the AOER calculation? Where source data makes it practical, the AOER takes the time-shifting effect of storage into account when calculating the emissions factor. When grid-scale storage charges, it is considered a load on the grid (not negative generation), so it is excluded from the total generation denominator of the calculation. When storage discharges, it is counted among the generation sources with a carbon intensity that is estimated to reflect the average AOER from prior times when it was charging. Sometimes, storage dispatch is not reported separately from other generation, and in those cases, we may estimate the disaggregation of storage.

Methodology

In the simplest case, the AOER is calculated by summing the total emissions from all generators in a particular hour and grid region, and dividing by the total energy generated (MWh). The steps below describe how publicly available data sources are used to perform a very basic calculation of AOER:

Step 1: Find the total generation in the region for each fuel type for a particular hour (e.g. reported generation by fuel type from EIA, ENTSOE, or similar).

Step 2: Assign annual CO2, CH4 and N2O emissions factors for each fuel type (e.g. use IEA annual factors by country).

Step 3: For each fuel type, multiply its total generation (MWh) by its annual emissions factor (lbs/MWh) and sum the results to get the total emissions (lbs); then divide total emissions by total generation to get the AOER (lbs/MWh) for that grid in that hour.

The above methodology produces a generation-based (aka “production-based”) average emissions factor. In a grid with little energy storage and no import/export (e.g., the Philippines), this factor is approximately equivalent to a consumption-based factor, as described by GHGP Scope 2 Guidance. The guidance says that the highest quality factor is a consumption-based factor which “should reflect net physical energy imports/exports across the grid boundary.” Further, the Scope 2 proposal released in 2025 states, “Consumption-based averages reflect electricity imports, exports, and stored electricity produced in a previous time period, whereas production-based averages do not.”

To comply with Scope 2 Guidance and the 2025 proposal, as of model date 2026-04-01, where there is significant import/export and storage, we also account for those using these additional steps:

Step 4: Using interchange data and flow-tracing (following the method of Hörsch et al., 2018), we determine the proportional fuel mix of electricity imported from each neighboring electricity grid.

Step 5: A new calculation of total emissions divided by total generation is performed, this time including both the local and imported generation mix, to get the “flow-traced” AOER. The volume of each fuel type is multiplied by the carbon intensity of that fuel type to estimate total emissions, which is then divided by the total generation from all fuel types.

Step 6: The carbon intensity for storage is then calculated. We perform a first-pass calculation where storage is excluded from emissions calculation; we then calculate the mean AOER in each region over a year during times when storage is charging, weighted by the volume of charging in each hour. That average is then saved as the carbon intensity for storage in that region, and the data is re-populated using the carbon intensity for storage in the total emissions calculation.

The AOER calculated in step 6 is the final AOER, which is published to the API as an hourly, consumption-based factor that accounts for import/export, storage effects, and non-CO2 GHGs as estimated CO2 equivalents.

Equation for Consumption-Based AOER

The final published AOER is an hourly, consumption-based emissions factor accounting for import/export, storage effects, and non-CO₂ GHGs as estimated CO₂ equivalents:

Variable Definitions

Symbol	Description	Units
	Final published AOER for the region for a particular period, e.g., hour (consumption-based)	lbs/MWh
	Generation from fuel type within the region	MWh
	Generation from fuel type imported from neighboring grid (flow-traced)	MWh
	Carbon intensity (annual emissions factor) of fuel type	lbs/MWh
	Energy discharged to grid from energy storage type	MWh
	Energy discharged to grid from energy storage type imported from neighboring grid (flow-traced)	MWh
	Flow-traced AOER from the previous year, volume-weighted over hours when energy storage type is charging	lbs/MWh
	Index over fuel types	—
	Index over neighboring grids	—
	Index over energy storage types	—

Transmission and Distribution Losses

WattTime’s AOER does not include transmission and distribution losses, so as to be aligned with the Scope 2 Guidance.

Some organizations may want to report emissions associated with transmission and distribution losses, for example for Scope 3 reporting purposes. T&D losses are the portion of source energy that is lost through T&D between the point of generation and the point of consumption. Below are a list of sources we recommend using for loss rates:

The EPA’s eGRID Dataset provides “Gross Grid Loss” (GGL) estimates for the three US interconnects, in addition to Alaska and Hawaii. This value includes both transmission and distribution losses, and is suitable for Scope 3 Reporting under the Greenhouse Gas Protocol.
The World Bank estimates country specific T&D loss rates for many countries in the world. This data is freely available.
The IEA also provides country specific T&D loss rates for many countries in the world as part of their annual “emission factors” dataset. This is a paid dataset with coverage largely equivalent to the World Bank.