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Deciphering the HERS Index: Or Trying To

We’ve been having a very interesting discussion on residential energy indexes like the HERS Index, Energy Score, and others. Some of the participants are advocating for a more direct and transparent number to present to homeowners that represents the energy efficiency of their homes.
I offered the following ideas as information about how we refer to energy benchmarking in commercial and multifamily buildings. I believe these simple metrics would work as a way to talk to homeowners about energy consumption, and how much it costs to reduce it.
Differences from the HERS Index

For commercial energy audits, we convert the kWh/yr and other energy usage to BTU/sf/yr and have a comprehensible scale of building energy consumption. I believe a scale spanning 20,000 to 300,000 would encompass 95% of all residential and commercial buildings. Some people use BTU/sf/HDD (HDD = heating degree days), but that assumes that heating is the major cost for buildings, which very often isn’t the case. You could drop the three zeros and say that the range is 20k to 300k (kBTU/sf/yr). Energy managers use that kind of metric frequently.

The major existing-building energy-management standards on the commercial side are ASHRAE 90.1 and ISO 50001. One idea that seems solid in these commercial-building standards is to pick a simple benchmark number like BTU/sf/yr and pledge to make that number go down. You have a plan for continual improvement that is outlined in your commercial energy audit. The process of energy management must be ongoing according to these international standards.

Saturn has a chart in our Energy Auditor Field Guide that compares the HERS index with BTU/sf/yr and kWh/sf/yr. It’s not approved by RESNET or anyone else. It’s just our way of struggling with the opacity of the HERS index. Here it is:

The Europeans talk about energy usage (kWh) as well as the cost of energy retrofits (Euros) “per square meter of floor space”. I find this helpful but I use BTUs, square feet, and dollars. Here is an example of how you can word a major energy retrofit for a customer. “I believe that you would need to spend more than $55/sf to bring your home from 80k to 20k.”

Hey, we’d love some feedback on the chart shown here!

 

 

2 thoughts on “Deciphering the HERS Index: Or Trying To

  1. John, I find this very illuminating. Especially for a visually oriented guy like me. I like that it breaks down to a metric not too far off of MPG.

    One question, is there a good unit that can be converted for any fuel type, i.e. coal, electricity, gas, fuel oil, etc? Are BTUs a good candidate?

    Thanks for posting!

    1. For most estimations a person might just say kBTU or megajoule (0.95 kBTU). Then the unit would be understood by energy geeks the world over. We have that comfortable range of 20 to 300 kBTU per square foot annually. A megajoule is around 95% of a kBTU. Incidentally this range would translate to around 205 to 3100 annual megajoules per square meter (1 square meter = 10.76 square feet).

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