EROCI Definition

EROI tells you how much energy you get back per unit invested. But it says nothing about carbon. A coal plant with an EROI of 30:1 and a wind farm with an EROI of 20:1 look comparable on energy return. They are not comparable on climate impact.

EROCI (Energy Return on Carbon Invested) measures how much useful energy a technology delivers per tonne of lifecycle CO2 emitted. It combines EROI's energy-efficiency logic with lifecycle emissions data into a single ratio. A technology with high EROCI produces lots of energy for very little atmospheric carbon.

Calculate EROCI for two sources. Nuclear produces roughly 83 MWh per tonne of lifecycle CO2 (based on ~12 gCO2e/kWh). Coal produces about 1.2 MWh per tonne (based on ~820 gCO2e/kWh). Nuclear's EROCI is roughly 70x coal's.

Now compare nuclear and natural gas. Gas lifecycle emissions run about 490 gCO2e/kWh, giving an EROCI of roughly 2.0 MWh per tonne CO2. Nuclear still delivers about 40x more energy per unit of carbon.

EROI ranks nuclear and gas within a factor of two (40-75:1 vs. 12-30:1). EROCI ranks them 40x apart. Why the divergence?

EROI ignores what kind of energy goes in and what comes out atmospherically. Gas has decent energy return but combustion-heavy emissions. Nuclear has excellent energy return and negligible operating emissions. EROCI captures a dimension EROI is blind to: how much useful energy a technology can deliver within a carbon budget.

For anyone evaluating decarbonization pathways, EROCI reframes the question from "how much energy does this produce?" to "how much energy does this produce per unit of climate damage?"

---