Grid Emissionality 2D: The REC Accounting Gap

Grid Conditions

Grid demand 900 MW

Wind availability 55%

REC purchase 0 MW-eq

1:1 certificate retirement (blocks double-counting)

Live Dispatch

Wind Solar Nuclear Coal Gas CCGT Gas peaker

Marginal plant (sets the price & the last MW)

—

Marginal emissions

0 gCO₂/kWh

Grid average emissions

0 gCO₂/kWh

Claimed avoided CO₂ (average method)

0 kg/h

Real avoided CO₂ (marginal method)

0 kg/h

Certificates issued vs. actual wind generation

100%

How it works

Grid operators dispatch power plants in merit order — cheapest marginal cost first — until cumulative output meets demand D:

sort plants by cost ascending
for each plant p: dispatch min(cap_p, remaining demand)
marginal plant = last plant whose output > 0
e_avg  = Σ(output_i · ef_i) / Σ(output_i)
e_marg = ef of the marginal plant

Wind and solar have ~$0 marginal cost, so they are dispatched first, regardless of who buys their Renewable Energy Certificates (RECs). The plant that clears the last MW of demand — usually gas or coal — is almost always the marginal plant.

Two very different accounting answers follow:

Claimed avoided CO₂ (the industry-standard "average" method): REC MWh × grid-average emission factor. This is what shows up on a corporate sustainability report.
Real avoided CO₂ (consequential / marginal method): the actual change in emissions caused by the REC purchase. Since the wind farm generates the same electrons whether or not you buy its certificate, and the marginal fossil plant still runs to cover the rest of demand, the real effect is close to zero — the emissionality gap.

Turning off "1:1 retirement" lets the REC slider exceed the wind farm's actual output this hour — a simplified model of double-counting / over-issuance, a real failure mode Guarantee-of-Origin registries are built to prevent by serializing and retiring each certificate exactly once.

Reading this 2D view: the top-left ring is the plant fleet — drag it to rotate. Flowing dots are physical MW; their color is each plant's emission factor. The top-right ladder is the merit-order dispatch stack, cheapest plant first, with the marginal (price-setting) block outlined. The bottom lane shows REC certificates flowing from the wind farm to a registry vault — a token flow that is completely decoupled from the physical power flow above, which is the whole point of the gap.

🔌 drag the ring to rotate · dots = physical MW flow · squares = REC certificate flow (decoupled from physical power)