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Building Science

New Mexico Develops Innovative Water Efficiency Rating Score

Like a HERS rating, the WERS rating will be scored on a 0 to 100 scale

Image 1 of 2
A WaterSense dual-flush toilet can help you get a low WERS.
Image Credit: Image #1: Kohler
A WaterSense dual-flush toilet can help you get a low WERS.
Image Credit: Image #1: Kohler
The WERS spreadsheet includes several tabs. This one shows the main inputs for the WERS, including rainwater and greywater reuse.
Image Credit: Image #2: WERS

Energy usually gets top billing in the green building community. It has a huge impact on the environment. We sometimes pay a significant amount for it (although most of us don’t pay enough to motivate serious change, but that’s another story). We can do energy modeling and home energy ratings. Plus, it’s just really interesting!

But water deserves a lot of attention, too, and green builders in New Mexico are innovating a way to move water to the fore.

Meet the WERS

If you’ve been reading this blog for any length of time, you’ve no doubt heard of a HERS rating. It stands for Home Energy Rating System, and it includes calculations that produce a HERS Index, a number whose main reference point is 100. The lower the number, the more energy efficient the home. A HERS Index of zero would be a net zero energy home.

Likewise, the WERS stands for Water Efficiency Rating Score1 and has a 0 to 100 scale, with lower numbers being better. Created by the Green Builder Coalition in cooperation with Build Green New Mexico (BGNM), Santa Fe Area Home Builders Association (SFAHBA), and members of the City of Santa Fe Water Conservation Committee (SFWCC), the WERS is a software-based program that tells you how efficiently your home uses water.

You may be wondering why they felt the need to create this when the US EPA already has a water efficiency program, WaterSense. There’s a big difference between the two, though. WaterSense is actually two things. First, it’s a label for products like toilets and faucets, similar to the Energy Star label for things like computers and refrigerators. Second, it’s a program for new homes. (Check out program details at the WaterSense website.)

So, the WaterSense new homes program is based on a checklist. WERS is a rating system, similar to HERS.

Calculating a WERS

The New Mexico folks have done a lot of work to put this rating system together, and it’s in the pilot stage now. The inputs and calculations are done in a spreadsheet (see Image #2, below), and it’s not that difficult to use. In fact, it’s easier than doing a home energy rating because you have a lot fewer inputs.

In Image #2, which shows one of several tabs in the WERS spreadsheet, you can see the inputs needed for the calculation. For toilets, faucets, and other indoor water appliances and fixtures, you enter the water consumption rate. The spreadsheet then calculates the water savings for each one compared to the industry baseline and prescriptive path requirements for water efficiency programs.

The “Minimum Prescriptive Indoor WERS” box at the bottom left shows the WERS for a home with the same inputs as you see in the “Program Minimum Prescriptive Path Units” column. In the example above, that would be 85.4. The box to the right of that shows the WERS for the home as rated, using the numbers in the “Proposed or Actual Units” column.

Currently the WERS includes only indoor water use, but it can be reduced by reusing greywater or rainwater. There is a checklist, however, for outdoor water use, and the plan is eventually to incorporate it into the WERS, too.

But doesn’t a HERS rating include water?

The short answer is no. Not like the WERS, anyway. Yes, a HERS rating includes hot water, but it’s only the energy use associated with it. And it doesn’t include anything about water use in toilets, faucets, and other water fixtures. The HERS rating is just about energy.

Once the new ANSI standard HERS protocols go into effect, the HERS rating will be better about hot water use. According to hot water efficiency guru Gary Klein, the thinking behind modeling hot water in a HERS rating now has shifted to put water efficiency and time efficiency (how fast does it get to the fixture?) first and the energy impact second.

But it’s not a WERS.

Scaling water efficiency

One of the best things about the WERS is the 0 to 100 scale. As with the HERS Index, it gives builders, home buyers, and anyone else who’s interested a way to compare houses. Kim Shanahan is Executive Officer of the Santa Fe Area Home Builders Association and one of the people responsible for making WERS a reality. “The zero to 100 scale of the WERS program plays right into the competitive nature of builders and the marketplace,” he said.2

The fledging program in New Mexico is off to a great start. There’s a lot of interest in it there, and they’re also having discussions with the EPA and others about taking it national. I think you’ll be hearing more and more about WERS in the coming years.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.


1. There’s one thing I wish they had done differently, though. WERS stands for Water Efficiency Rating Score. (At least that’s what it is in most places I’ve seen it defined.) What that means is that it’s going to become the next Grrrrrrrrrr-inducing term like ATM, PIN, and ECM because everyone is going to want to say WERS score the way they say ATM machine, PIN number, and ECM motor. I’m feeling it already. Grrrrrrrrrr!

2. Yes, “he.” Kim is a perfectly respectable man’s name. Which reminds me: In case you haven’t heard, I’ve started a new movement to get parents to reclaim names that were traditionally boys’ names but which have since been co-opted by girls. Come on, parents! Don’t be afraid to name those boys Allison or Kim or Kelly or Mary Ann. Well…maybe not Mary Ann, but did you know that John Wayne’s original name was Marion Robert Morrison?


  1. GBA Editor
    Martin Holladay | | #1

    Outdoor water use
    Thanks for reporting on this issue; good job.

    I'm curious to know whether WERS makes any attempt to quantify outdoor water use -- that is, water used for lawns, landscaping plants, and swimming pools. When it comes to residential water use, those are the big-ticket items -- not toilets or showers.

  2. GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Martin Holladay
    Martin, yes, there's a checklist for outdoor water use, but it's not used in calculating the score. I've been told, though, that it probably will be included eventually. You also get credit in the WERS for reusing rainwater.

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