Average vs Marginal Emissions

If you charge an EV in France (mostly nuclear), it adds almost no emissions. If you charge it in Poland (mostly coal), it adds a lot. But the real question is not what the grid's average looks like; it is what generator turns on or off in response to your additional demand. That is the marginal emission factor.

The average emission factor divides total grid emissions by total generation: gCO2/kWh across all sources. The marginal emission factor asks a different question: when one more kWh of demand is added (or removed), which generator responds? On most grids, the marginal generator is a natural gas plant, because gas ramps fastest to meet demand changes. This means that charging your EV at midnight likely causes gas-fired generation to increase, even if the grid's average intensity is low because of nuclear or hydro baseload.

Calculate both factors. A grid generates 60% from nuclear (12 gCO2/kWh) and 40% from gas (490 gCO2/kWh). Average intensity: 0.60 x 12 + 0.40 x 490 = 203 gCO2/kWh. But gas is the marginal source, so the marginal emission factor is closer to 490 gCO2/kWh.

Apply to EV charging. Using the average factor, your EV "emits" 203 g/kWh. Using the marginal factor, it causes 490 g/kWh of additional grid emissions.

Which factor should a policymaker use to evaluate the climate benefit of EVs?

It depends on the question. For carbon footprinting (what is the EV's share of existing emissions?), use average. For evaluating the impact of a new policy or technology (what additional emissions does it cause?), use marginal. Using the wrong factor produces misleading conclusions.

Choosing between average and marginal emission factors is one of the most consequential methodological decisions in climate policy analysis.

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