Avoided Emissions

The C.Scale model calculates the avoided emissions associated with surplus energy generated on-site through photovoltaic panels. The benefits from exported utilities (e.g., the avoided impact of grid electricity generation) are reported in Module D and as a separate model element called “Avoided Emissions.” Module D must be communicated separately from A-C impacts since it represents benefits outside of the system boundary of the building project.

Calculation of avoided emissions from surplus energy generation

Once production from an onsite solar array has exceeded annual electricity use, C.Scale assumes all additional energy generated by the array displaces the generation of electricity by the electrical grid. The avoided emissions from surplus onsite energy generation are calculated as the emissions that this displaced energy would have incurred.

This method assumes that there is no curtailment of PV production and that the carbon emissions of grid electricity when solar energy is produced are substantially similar to the annual average emissions. In locations with a high proportion of solar on the grid, these assumptions will not hold, and skepticism of C.Scale's calculation of avoided emissions is warranted. Read more about curtailment in this publication (pdf) from the National Renewable Energy Laboratory.

The assumption of displacement generation will not hold true in all locations, and some skepticism of this estimate is encouraged. Two interrelated situations where C.Scale's assumptions of displaced generation will not hold are when:

• Daytime (i.e., when solar is available) emissions from the electrical grid are significantly lower than the national average.

• Surplus energy generation is expected to be curtailed by the utility. For an overview of curtailment in the United States, we recommend this report from NREL (pdf).

The generation of excess energy by an onsite solar photovoltaic array displaces the generation an equivalent amount of electricity from the utility grid. This is calculated as follows:

$$Avoided\ carbon\ emissions=\ \sum_{t=1}^{m}\ e_{t}\ast c_{t}$$

Where $e_{t}$ is the excess energy in kWh generated in year $t$ and $c_{t}$ is the carbon intensity of the electrical grid per unit demand in year $t$. This method assumes that there is no curtailment of PV production, and that the carbon emissions of grid electricity when solar energy is produced is substantially similar to the annual average emissions. In locations with a high proportion of solar on the grid, curtailment is likely and skepticism of C.Scale's calculation of avoided emissions is warranted.

If you are interested in further analysis of hourly emissions, please reach out.