A New Tool for Measuring Decarbonization

The RESNET CO2 Index can allow government and utilities to set targets for carbon emissions reductions in homes based on site-specific metrics for carbon. Homeowners can also use this information to lower their carbon footprint.

(c) David B. Goldstein 2022

Climate change is mostly caused by carbon dioxide emissions, and in the U.S. some 35 percent of these emissions result from burning fossil fuels to power, heat, and cool buildings. Decarbonization of our buildings is critical if we hope to meet the climate goal of reducing U.S. emissions by well over half by 2030. But until now we have lacked an accurate tool to calculate the carbon impacts of an individual house or building and the savings we are achieving through clean energy measures. Thankfully, RESNET, a nonprofit standards development organization has developed such a tool.

Climate change threatens our health and our economy.

Increasingly, climate change affects each of us individually. I experienced this on September 9, 2020 when this view confronted me from my home: a dark orange sky (the picture shows the true color) that made midday look like late twilight.


But things got worse, as the next day the smoke—from wildfires 500 miles away linked to climate change—descended to breathing level and produced some two weeks of the unhealthiest air on earth extending from north of Portland, Oregon to Los Angeles. Climate change is not a distant threat affecting  strangers across the ocean. It is a crisis that has already arrived on most of our doorsteps. And if it hasn't affected you personally yet, it likely will in the next 5 or 10 years.

Reducing Buildings’ Contribution to Climate Change

Improving our buildings—and other parts of the economy—to reduce or eliminate these emissions is called “decarbonization”.

Decarbonization of our buildings is critical if we hope to meet the climate goal of reducing U.S. emissions by well over half by 2030.

It can be achieved through:

  • increasing the energy efficiency of our buildings,
  • adding renewable energy to a facility or the grid,
  • changing the time at which electricity is consumed so that most, or all of it, can be supplied by renewables—like solar and wind power, and
  • Changing from a fuel with higher emissions to electricity

Until now, we have lacked a tool to tell us exactly how much carbon emissions we are saving and that allows us to compare the effectiveness of one decarbonization  measure over another. Even experts have been forced to rely on generic studies of average emissions over a year; or inaccurate assumptions that carbon emissions are proportional to energy consumption.


(c) David B. Goldstein 2022

A RESNET energy rating label for a highly efficient home

We now have a new tool to help decarbonization

RESNET, a nonprofit standards development organization that oversees the use of the ENERGY rating index, recently released an ANSI standard on how to calculate the carbon impacts of an individual house or building. It is called the RESNET "CO2 Rating Index". The standard is among the first in the world, if not the very first, to estimate emissions accurately by:

  • accounting for the hour of the day and month of the year at which electricity is consumed,
  • using the incremental effect that the building has on usable renewable energy and on fossil fuel emissions, calculated over the long term (so that the additional solar or wind that is built in response to the building is counted).

This standard allows anyone to calculate the benefits of electrification for any residential building anywhere in the United States, using inputs customized for any chosen need. (Other countries can use the same methods to calculate emissions from their grids.)

It offers benefits to:

  • jurisdictions that want their energy codes to produce the maximum carbon savings
  • utilities that have carbon reduction goals
  • home owners or renters who want to reduce their carbon footprint

It’s critically important to calculate emissions on an hourly basis because as solar and wind become the backbone of our electric system, the cleanliness of the system varies with time of day and time of year.  We have seen this outcome in leading states such as California, and it will soon follow everywhere else, as jurisdictions and corporations adopt targets for renewable energy. The next two figures show how much difference this change makes:


* CPUC Avoided Cost Model 2018: http://www.cpuc.ca.gov/General.aspx?id=5267

Average Hourly Grid Emissions Profile, Full Year; Source: Brook, M. 2018. Building Decarbonization. Docket 18-IEPR-09. Sacramento, CA: California Energy Commission

To understand the second graph, see how the hour of the day is given by looking up or down a column, while the month of the year is given by moving across a row. Dark green indicates LOW carbon emissions and dark red means HIGH emissions. The first figure shows that even in 2019, electricity used in the late part of a summer day was five times more polluting than during the middle of the day. The second figure shows that the difference will get more intense by the end of the decade as more renewables displace fossil fuels—when the wind is blowing or the sun is shining.

Energy ratings already look at the patterns of energy use by the hour of the year. So it is a straightforward task to translate the energy profile into an emissions profile if you have the data that underlie these figures. The basic analysis that created the data was recently completed by the National Renewable Energy Laboratory and is incorporated into the RESNET standard. It covers twenty different geographic areas in the U.S. with varying use and plans for renewable electricity production.

That is why the new RESNET CO2 Rating Index is so exciting. It allows us—right now—to calculate the comparative emissions of two houses. For example, we can look at the emissions from a new house with, or without, efficiency and solar to see how much electrification and clean energy can help. Or we can compare a house as-is with how it would look with a deep decarbonization retrofit.

And it will get better than that. Once we see that emissions depend on WHEN electricity is used, we can start to take credit for actions that change the time of use. One good example of this is the heat pump water heater, which can be programmed or told via the Internet to raise water storage temperatures when we are in a green hour on the graph and draw down the stored heat during a red hour.

Another great example of a “demand shift” technology is electric vehicles, which can be charged exclusively during the green hours and discharged partially during red hours.

Standards on how to credit these techniques are high on RESNET’s priority list for future actions in 2022.

Action Items

It is commonly said that “you can’t manage what you can’t measure”. Up until now, we have not been able to measure the carbon footprint of houses, which account for some 15% of ALL U.S. emissions, on an actionable level. Now we have that tool and can accelerate our efforts to decarbonize buildings based on the facts.

This tool is not even limited to houses: the analysis behind it applies to almost any use of electricity. So with minor adaptation, the RESNET standard can help decarbonize commercial buildings and industry as well.

The ways to do this are direct and simple. The index can be used as a supplementary requirement in energy codes and specifications, allowing the most flexibility to the designers and builders while requiring specified levels of carbon emissions savings. The codes and spec’s can apply both to retrofits as well as to new construction. We hope to see model codes and specifications such as EnergyStar begin to make use of the RESNET CO2 Rating Index soon.

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