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How to actually quantify the energy savings in a green building?

SR Valluri | Posted in Energy Efficiency and Durability on

We say that a green building consumes say 20 or 30% lower power consumption when compared to a conventional building. How to ratify this? How can we measure and show the difference?


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  1. GBA Editor
    Martin Holladay | | #1

    Your question raises further questions:
    1. What is a baseline building? Most people would answer that a baseline building is one that barely meets minimum code requirements. Such a home may be much worse (or, more rarely, better) than the "typical" new home.

    2. Once a baseline has been established, there are two ways to rate a new energy-efficient building: (a) using computer modeling, and (b) using 12 months of actual monitoring data (or utility bills). While method (a) is commonly used, the method is controversial --because actual energy consumption is often significantly higher than computer modeling would indicate.

    In other words, there are at least two commonly used methods of exaggeration. The first method is to choose an unrealistic baseline -- an energy hog house. The second method is to choose an unrealistic (optimistic) method of estimating energy use in the building that one wants to brag about.

  2. SR Valluri | | #2

    One method is choosing the unrealistic baseline and the other is choosing unrealistic method of estimating energy use.
    That means we can not get the actual savings. It depends on assumptions which are made. I feel it is extremely difficult to establish the savings claimed.
    Ideally speaking if two buildings are constructed exactly similar to each other one conventional and the other a Green building then we can get the actual energy savings. But this is a hypothetical proposition. I feel that there will be a gap between the actual and claimed energy savings.Pls comment


    It is further complicated by the occupancy of the house. If you work at home and keep a couple of computers, coffee maker and so on running all day your home will use more energy than if you drive twenty miles to a minimally insulated office park. Of course you are eliminating the commute and the utilities at the office park so it's a net positive.

    When my teenage daughter went off to college my hot water and electric (and phone) usage went down.

    Not only is the baseline elusive but the actual usage is quite variable from year to year. I recently had one of my houses double in energy consumption when it was rented for a year to occupants who were un-educated about living in a solar house. A good owners manual can improve the energy (and durability) performance of the house, how do you calculate for that?

  4. GBA Editor
    Martin Holladay | | #4

    Study after study has confirmed that actual savings attributable to energy-efficient retrofit work or energy-efficiency features included during new construction are significantly less than projected or modeled savings. Several factors contribute to this finding, including problems with computer modeling software, discrepencies between specifications and the as-built reality, and that old bugaboo, the Occupant Problem.

    It's easier to build an energy-efficient thermal envelope than to build an energy-efficient occupant. As we all try to tackle the global climate change problem, we're going to need both.

  5. Riversong | | #5

    First, we need to differentiate between a "green" building (which does little harm to the environment) and an energy-efficient one (which is what the euphemism "green" normally refers to). You're speaking of energy-efficient buildings.

    Second, the only way to compare buildings is by computer modeling, which excludes the uncontrollable and infinitely variable behavior of the occupants. This is identical to the way vehicle mileage is compared - independently of who drives the car.

    The EPA Energy Star program is an excellent method of quantifying the relative energy performance of residential buildings. It uses as a baseline a new house built to the minimum standards of the International Energy Conservation Code.

    The Home Energy Rating System (HERS) number is the percentage of total energy (heating, cooling, lighting, appliances) that a house is modeled to require as a percentage of the IECC home, which is given a HERS 100 (100%).

    An Energy Star home must be modeled to require no more than 85% or 80% of the IECC standard home, depending on climate zone. This is an objective metric which allows comparison between different houses.

    Using actual occupancy consumption is far more a measure of human behavior (either frugality or, more typically, extravagance) than it is a measure of the efficiency of the building.

    And net-zero homes (HERS 0), are not necessarily energy-efficient homes - merely homes that use expensive and resource-depleting technologies to produce as much energy as they consume. Thus they should not be compared on the same scale as other homes unless only their consumption is modeled.

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