03 February 2012

Net Zero

(All photos from the Sol blog.)
This week the New York Times had an article about Sol, a modern and very green development on the outskirts of Austin.  (Sol, by the way, is both the Spanish word for "sun" and also an acronym for "Solutions-Oriented Living"...clever, no?)  The article is well worth a read, not only for the specifics of the homes' energy-efficient features and the interesting history of the development through the ups and downs of the real estate market, but also because of how it addressed the concepts of "net zero" and "net zero-capable."  (Apparently Sol had initially marketed itself as Austin's first net zero community, but the economic downturn led the developers to abandon that goal.)

According to the article, the U.S. Department of Energy defines "net zero-capable" as a house that uses 55% less energy than a typical house built in 2006.  The article also mentioned that the City of Austin has a goal of making all new construction in the city net zero-capable by 2015, which caused us to wonder how that concept applies to large homes (is it based on the average energy use of all houses built in 2006?  Scaled to the size of the house?), but I was unable to find anything on the Department of Energy website to explain what 2006 house or houses they use to set that standard.

Anyhoo, all Sol houses are net zero-capable, with energy-efficient windows, spray foam insulation, well thought-out design to deal with Austin's heat, and other green features.  However, only one house of the eleven that have been built so far is actually net zero(ish), with a geothermal system that reduces its energy usage and solar panels to supply nearly all of the power.

Net zero(ish) isn't exactly a technical term, but it's an important term to me because that's exactly how I would describe our house.  I'm pretty sure we'll still have to buy some electricity at certain parts of the year -- although I don't know for sure because we still haven't received a bill since our solar panels started up in October (ongoing billing system glitch) -- but since we won't owe anything most months, I think our house definitely qualifies as net zero(ish).  (What we do know is that, since our panels started generating on October 20, we've put about 150 more kilowatt-hours into the grid than we've taken from it.)

But back to Sol.  While the modern aesthetic wouldn't be my first choice, I really dig the way the homes all have these courtyards to maximize the living space (the houses are tiny -- most of the floor plans are under 1,500 square feet).


  1. I'll wager that y'all's house is net-zero: generates as least as much electricity that it uses (can't remember if y'all have a gas stove or not; that complicates the equation a wee bit...). Net-zero capable is a funny term to me... At least with Sol, the homes are solar ready (in addition to efficient), so they could (actually-possibly) be net-zero.

    Sol is cool! The builder for Sol is who we hope to use for our house!

    1. Good point. We do have gas for our range, as well as our fireplace (not that we've ever used it) and our grill, but that usually amounts to one unit of gas a month (Thanksgiving actually bumped November up to three units). I suspect if we had a really cold, cloudy winter month, we'd come up way short on electric, and we have yet to see how we make out in the summer, but yes, I'd like to think we're net zero...ish.

      Sol IS cool. : )

  2. Your question about the 2006 standard has to do with the HERS index. HERS stands for Home Energy Rating System and it's what the DOE uses to give houses the Energy Star label. It's not based on any single house actually built in 2006, but rather the standards and specifications defined in the 2006 edition of the International Energy Conservation Code (IECC). This, then allows one to estimate what the energy use of that house would be.

    Energy Star has been around a lot longer than 2006. Version 1.0 used a 1993 energy code as a basis for comparison, but the latest versions (2.0, 2.5, and 3.0) still use this 2006 benchmark for comparison. It appears that the 2006 benchmark is here to stay, which is a good thing for the sake of keeping a consistent reference point for comparison.

    The HERS index uses a 1 to 100 scale which represents how much energy a house uses relative to a comparable house built to the 2006 benchmark. A HERS index of 100 means that the house in question uses 100 percent as much energy as its 2006 counterpart. A HERS index of 50 means that the house in question uses half as much, and a HERS index of 0 essentially means net-zero. HERS indexes can go into negative numbers which mean the house in question is a net energy exporter.

    The HERS index is flexible enough to account for the size of the house. Version 3.0 includes a square-foot conversion chart based on number of bedrooms, but I don't quite understand that yet. The best figure for comparison is energy intensity, which breaks down energy use or demand by square foot, e.g., kWh/s.f. or BTUh/s.f.. I've looked around for HERS index-to-energy intensity conversion, but so far, haven't had any luck. One basic scale that I found here at www.energyvanguard.com/blog-building-science-HERS-BPI/bid/20376/What-s-Your-Number gives you a general understanding of your home's relative energy efficiency based on its energy intensity. This is just that person's scale, but it will give you a sense of how you compare, for what it's worth.

    Since Energy Star and Resnet use the 2006 benchmark, it has become sort of an industry standard. To give you a sense of how the HERS index compares, here are a few figures:
    An average home built before 2006 is around HERS 130
    The benchmark to qualify as an Energy Star Home (v2.0) requires a HERS index of 85 Houses built to the 2009 IECC (adopted in Austin in 2012) are estimated to be in the HERS 85 to 80 range (although recent DOE data suggests that to be more in the 90 to 95 range)
    Houses built to the 2012 IECC (not yet adopted anywhere) are estimated to be around HERS 70
    A LEED certified home would earn a HERS index of 40 to 70.

    So, these houses at SOL are doing pretty good, by comparison. According to the article, the houses at SOL would get a HERS index of 55. That is certainly better than current codes and existing houses and puts them in the same category as LEED certified homes - definitely a big step in the right direction.

    The whole "net-zero capable" term is a little troubling to me, though. If you think about it, any home is "net-zero capable", it's just a matter of how much money you're willing to spend to get it to that point. Of course, if your house is an energy hog, has poor orientation for PV, and other constraints, that might make it "net-zero INcapable", it's still a ill-defined term that shifts the responsibility from the builder/developer to the owner. In the case of SOL, it's the result of a compromise where market conditions turned good intentions into a marketing term.

    As for your house, and as I'm sure you already know, it won't be until after a whole year, that you'll know whether or not you're truly net-zero. Winters are mild, but summers are brutal.

    1. Thanks for sharing all of that. (I'd love to transition out of law and into something involving green building, utilities, etc., but I'm woefully uneducated about the technical stuff.) We finally got our first utility bill over the weekend -- for the first three months of solar -- and I'll be writing about it soon, but the bottom line is I'm actually more confident that we'll be net-zero once summer is factored in -- with such long days and good orientation (plus great insulation, windows, overhangs, geothermal HVAC, etc.), I anticipate that summers will give us the biggest surplus (as we had in October, before the cool, cloudy days started). Time will tell....