Marginal emission factor is the carbon intensity (e.g., kg CO₂e per kWh) of the next unit of electricity generated to meet an additional change in demand on the power grid. In other words, it estimates the emissions impact of consuming (or saving) one more kWh at a specific time and location, based on which power plants ramp up or down at the margin.
What Is a Marginal Emission Factor?
A marginal emission factor (MEF) answers: “If I add 1 kWh of load right now, what extra emissions does the grid produce?”
It differs from an average grid emission factor, which spreads total system emissions across all kWh produced in a period.
MEFs are typically time-dependent (hourly/seasonal) and can be location-specific, because the marginal generator changes with demand, renewable output, imports/exports, and grid constraints.
Why Marginal Emission Factor Matters in EV Charging
EV charging is flexible in many scenarios (workplace, residential, fleet depots). Using MEFs enables carbon-aware charging by shifting charging to times when the marginal electricity is cleaner. This supports:
– Lower charging carbon reporting emissions for users and fleets
– Smarter managed charging decisions beyond cost-only optimization
– Better alignment with corporate decarbonization and ESG reporting goals
– Grid-friendly behavior by avoiding high-emission peak periods
For CPOs and fleets, MEF-based optimization can enhance the credibility of CO₂ reduction claims compared to using only annual-average factors.
Marginal vs Average Emission Factor
Marginal emission factor (MEF)
– Emissions from the incremental generator responding to demand changes
– Best for assessing the impact of changing load (shift, add, or reduce charging)
– Often varies significantly by hour
Average emission factor (AEF)
– Total grid emissions divided by total electricity delivered over a period
– Best for attributing emissions to total consumption over a year/month
– Smoother, less sensitive to timing
Both can be used in reporting, but they answer different questions: impact (marginal) vs allocation (average).
How Marginal Emission Factor Is Used
In practice, MEFs are applied to:
– Estimate emissions for flexible loads on an hourly basis
– Compare “charge now” vs “charge later” scenarios in smart charging
– Quantify avoided emissions from demand response or peak shifting
– Prioritize charging when renewables are likely to be on the margin
A common workflow for EV charging uses:
– Time-stamped charging energy (kWh) from the charger or CPMS
– A time series of MEFs (by region/grid zone)
– Multiplication per interval to calculate CO₂e impact
What Drives Changes in the Marginal Emission Factor
– Which generators are dispatchable and responding (gas, coal, hydro, imports)
– Renewable availability (wind/solar) and curtailment
– Peak demand periods and system constraints
– Interconnector flows between countries/regions
– Market dispatch rules and balancing actions
Because these factors change hour by hour, MEFs are usually higher during stressed periods and lower when clean generation is abundant—though this depends on the specific grid mix and dispatch.
Limitations to Consider
– MEFs depend on grid modeling or market data and can vary by methodology
– Spatial granularity may be limited (country vs bidding zone vs node)
– Not always appropriate for formal corporate accounting where specific standards require average or market-based factors
– Results can be sensitive to assumptions about which generator is truly “marginal”
Related Glossary Terms
Carbon-aware charging
Charging carbon reporting
Carbon intensity
Grid emission factor
Average emission factor
Managed charging
Smart charging
Demand response (DR)
Peak shaving
ESG reporting