A lot of home builders and homeowners are getting certified home energy ratings to find out how efficient their homes are. There's also a lot of buzz about HERS ratings, with builders looking at them as a tool for marketing their homes. As I described in our basic description of home energy ratings, this process models the energy performance of a home and comes up with, among other results, a number called the HERS Index. But what exactly is this thing called the HERS Index?
There are a lot of ways to approach that question. My objective here is to focus on the mechanics — how the HERS Index is defined and how it's calculated. For greater depth on the mechanics or more on HERS ratings in the broader context, see the links at the end of this article.
Here are the fundamental ideas behind the HERS Index:
The image below shows the scale for a home with a HERS Index of 65, which would be 35% more efficient than the Reference Home.
If the basic info above isn't sufficient for you, the HERS Standardswould be the ultimate place to go for more details. For example, if you wanted to see the actual equation used to calculate the HERS Index, you could go to page 3-4 in chapter 3 and find it. To save you the effort, I've pasted a copy of the equation directly from the HERS Standards below.
Yes, it's ugly, ugly, ugly! You could spend a while sorting through all the horribly-named variables, but, since you're reading this article, you can skip right over that bit of ugliness above (Have I mentioned how ugly it is?!) and get the idea behind it from my simplified version of the equation, shown below.
Much better, right? The E stands for energy consumption, WH stands for water heating, and LA stands for lights & (some) appliances. PEfracneeds a separate explanation, which I'll give you shortly. To be fair, though, there's still a lot hidden in both versions of the equation you see above. Calculating those four individual components (heating, cooling, water heating, and lights & appliances) involves a whole lot of definitions, specifications, assumptions, and yet more equations. Chapter 3 of the HERS Standards, which contains the technical standards for HERS ratings, is 44 pages long and is constantly being updated, revised, and amended.
The HERS Index equation basically compares the energy use of the rated home to the energy use of the HERS Reference Home. If the total of those four components is the same for both the rated and Reference homes, the fraction part of the equation equals 1. Then you multiply it by 100 to get a HERS Index of 100 in that case (assuming PEfrac = 1 for now). If the rated home's energy use is half of the Reference Home's energy use, the HERS Index would be 50. If the rated home uses twice as much as the Reference Home, the Index would be 200.
Now, what about the PEfrac part? First, the PE stands for 'Purchased Energy,' so that gives you some idea of what it does. It's a multiplier that can reduce the HERS Index for homes that produce some or all of the energy they use over the course of a year. Here's what the equation for PEfraclooks like, again simplified from the HERS Standards to make it more understandable:
If a home has no on-site power production (Eproduced = 0), the fraction is just 1 and it has no effect on the HERS Index. If the rated home produces an amount of energy equal to half of what it uses (Eproduced = 0.5 x Eused), PEfrac = 0.5 and it cuts the HERS Index in half. If the rated home produces the same amount of energy over the course of a year as it uses (Eproduced = Eused), PEfrac = 0 and the HERS Index is also 0. That is, it would be a net-zero energy home. If the home produces more energy than it uses (Eproduced > Eused), PEfracwill be negative, and so will the HERS Index.
One aspect of the HERS Index that bears further scrutiny is the HERS Reference Home. Here's what the HERS Standards say about it:
“The reference home is the geometric twin of the rated home, configured to a standard set of thermal performance characteristics, from which the energy budget, that is the basis for comparison, is derived.”
Basically, the Reference Home is the same size and shape as the rated home and is also in the same location and IECC climate zone. The inputs for insulation R-values, window U-values, HVAC system efficiency, and similar factors are defined in the HERS Standards but are close to what's in the 2006 IECC.
Let me point out a couple of important points about the Reference Home. First, some aspects vary with climate zone, and some are fixed. For example, wall insulation R-values change with location but the window area in the Reference home is always 18% of the conditioned floor area and is spread out equally on south, east, north, and west facing walls. Second, the Reference Home doesn't have everything that the rated home has. Although a rated home may have photovoltaic modules, for example, the Reference Home will not.
The really important thing to know about the HERS Reference Home is that it's your reference point, hence the name. Yes, the energy code that it's based on has advanced, but we don't want to go changing our reference because if we did that, the scale would change. That would be like changing the definition of the mile just because our modes of transport got faster.
Likewise, the HERS rater industry in Canada is just getting going and could adopt a different HERS Reference Home. If they want to be able to compare their homes' HERS Indices to ours in the US, however, they need to keep the same definition of the HERS Reference Home. Otherwise it's apples and oranges, and CRESNET's Cross-Border Challengewould be difficult to judge.
That's it. A HERS Index is a number that compares how a given home's energy use to a reference point, which is the HERS Reference Home. Numerically, it's like golf scores - the lower the number, the better. I've seen scores above 200 for really bad houses and a low score of -2 for a net-zero energy home in Tennessee.
The equations above show how to calculate it. As they always say, though, the devil's in the details, so if your intention is to write software to be used as an accredited HERS rating tool, you'll need to spend a lot of time poring over the HERS Standards. If you're just looking for the overview with some detail about what goes into calculating a HERS Index, I hope this article gave you what you needed. If not, leave a comment below.
How Is a Home’s HERS Index Calculated? by Martin Holladay at Green Building Advisor
Energy Modeling Isn’t Very Accurate by Martin Holladay at Green Building Advisor