Estimates of CO2 in Energy Stewards–Part 2

In the last post, we showed how Energy Stewards® uses numbers from ENERGY STAR to estimate CO2 impact and changes from a baseline year on the building home page and on the detailed graphs page.   Remember that for electricity, ENERGY STAR estimates the CO2 based on the location of your building, assigning average emissions from the eGRID sub-regions.

Map of electric grid regions

In Energy Stewards, there is one other place where you can look at estimates of CO2 associated with your building’s use of electricity and natural gas.

On your building’s home page, scroll to the bottom to see the thumbnail graphs of electricity and natural gas (if your building uses natural gas.)

You can choose to look at energy use for your building in energy units, in dollars, or in lbs CO2, all normalized by the area of your building.  Using the same choices of units, you can also compare your building in thumbnail graphs for electricity and natural gas.

For the thumbnail graphs, Energy Stewards also uses the eGRID numbers to estimate the CO2 associated with electricity generation. We use a state-based number based on your building’s address rather than the more precise match of building zip code to eGRID region used by ENERGY STAR.

For natural gas, we use 11.7 lbs CO2 per therm, which aligns with the CO2 calculation for natural gas used by ENERGY STAR, converting metric to ordinary units.

Again, as we said in the previous post, these CO2 estimates do not account for fuel extraction but only combustion of natural gas directly or for the combustion of the fuel to provide electricity to the grid.   The numbers we present should be seen as representing “at least this much CO2.”

(And as we noted last time, since natural gas is mostly methane (CH4) and methane per gram has 20 times the greenhouse gas impact as a gram of CO2, leaks from natural gas extraction are an important factor in total greenhouse gas accounting.  In fact, the leaks associated especially with hydraulic fracturing (“fracking”) make some analysts believe that the recent discoveries of lots of natural gas in the U.S. and other places is a net negative with respect to climate impact rather than a bridge to a less carbon intense future.)

If you have questions about carbon numbers in your building’s use of energy, let us know–we’ll work through the arithmetic with you!


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