The Residential Energy Services Network’s (RESNET) Home Energy Rating System (HERS) Index is the nationally recognized system for inspecting and calculating a home’s energy performance. Homes are rated for energy efficiency and issued with a HERS Index Score based on their energy performance. The rated home is compared to the HERS Reference Home to determine how energy efficient it is. The HERS Reference Home is modeled on an energy efficient home that conforms to the 2004/2006 International Energy Conservation Code (IECC) and has a HERS Index Score of 100. The lower a home’s HERS Index Score, the more energy efficient it is. So how is the HERS Index Score calculated?

The actual calculation is pretty complex and based on a number of variables taken from the HERS Standards (Chapter 3, page 3-4). A simplified, layman’s version that makes it a little more understandable is as follows:

 

resnet-hers-index-equation

 

Legend: E=energy consumption, wh=water heating, la=lights and some appliances, PE=purchased energy (fraction).

Basically, the way it works is the energy use of the rated home is compared to the energy use of the reference home. If the energy use for both works out to the same amount, then the fraction part of the equation (PEfrac) equals 1. Multiply 1 by 100 and in this case, you get a HERS Index Score of 100.

Now, if the rated home’s energy use works out to half of that for the reference home, its HERS Index Score would be 50. However, if the rated home uses twice as much energy as the reference home, then its HERS Index Score would be 200.

The Purchased Energy or PEfrac is a multiplier that can reduce the HERS Index Score for those homes that produce some or all of the energy they use over the period of one year. There’s a separate equation to achieve this number, which breaks down to something along these lines:

 

pefrac-equation

 

Legend: PE=purchased energy, E= energy

  • A home that produces no power (Eproduced = 0) has a PEfrac of 1 and doesn’t effect the HERS Index Score.
  • A home producing an amount of energy equal to half of what it uses (Eproduced = 0.5 x Eused) will have a PEfrac = 0.5, which will cut the HERS Index by half.
  • A home producing the same amount of energy as it produces (Eproduced = Eused) will have a PE frac of 0, producing a HERS Index Score of 0. This is a net-zero energy home.
  • A home producing more energy than it used (Eproduced > Eused) will have a negative PEfrac, and therefore a negative HERS Index Score.

That’s the science behind calculating the HERS Index Score, in a nutshell. Fortunately, issuing a home with a HERS Index Score doesn’t require the manual calculation! A certified RESNET Home Energy Rater will carry out a home energy rating and use RESNET Accredited Rating Software to calculate the score. To learn more about the HERS Index and energy ratings, contact a certified RESNET Home Energy Rater.

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