Carbon Accounting Equations for Energy Modelers

Carbon Accounting Equations for Energy Modelers

This reference sheet summarizes the key equations and data sources energy modelers need to convert energy use into carbon impact for compliance, decarbonization, and reporting.

1. Operational Carbon Emissions

Equation: CO₂e = Energy Use × Emission Factor

Fuel Type Equation Units
Electricity CO₂e = kWh × EFelectricity kg CO₂e
Natural Gas CO₂e = Therms × EFgas kg CO₂e
Fuel Oil CO₂e = Gallons × EFfuel_oil kg CO₂e

Emission Factor References:
- EPA eGRID (U.S. Electricity EF)
- EPA GHG Emission Factors Hub
- NRCan Emission Factors (Canada)
- GHG Protocol

2. Fuel Conversion (Energy Units)

From To Multiply by
Therms kWh 29.3
kBtu kWh 0.293
MMBtu kg CO₂e (gas) 53.06*

*53.06 kg CO₂e/MMBtu is U.S. EPA default for natural gas (2023).

3. Carbon ROI Equation

Carbon ROI = (Baseline CO₂e – Proposed CO₂e) / Embodied CO₂e

  • Baseline CO₂e: emissions of the original system
  • Proposed CO₂e: emissions after ECM or system upgrade
  • Embodied CO₂e: GHG impact of new equipment, materials, etc.

4. Modeling Output Integration

Model Output Source How to Convert
Electricity (kWh) eQUEST, EnergyPlus, IES VE × EFelectricity
Gas Heating (Therms/MMBtu) Energy Model Output × EFgas
Site EUI (kBtu/sf/yr) Summary Reports × Area → Convert kBtu to CO₂e

5. Example Calculation

A building uses 7,500 therms/year of natural gas.

CO₂e = 7,500 therms × 5.3 kg CO₂e/therm = 39,750 kg CO₂e/year

6. Embodied Carbon & Lifecycle Emissions

For whole-building carbon assessments, include both operational and embodied carbon.

Lifecycle Equation:

Total CO₂e = Operational CO₂e + Embodied CO₂e

Embodied Carbon Equation:

Embodied CO₂e = ∑ (Material Quantity × EFmaterial)

  • Common materials: concrete, steel, aluminum, insulation, glass
  • Sources vary by manufacturer and geography

Key Tools & Databases:

Pro Tip: Compare carbon ROI to energy ROI to evaluate both short- and long-term benefits of ECMs.