Green Topics

Want a Net-Zero Home? Be a Net-Zero Family

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By Martin Solomon and Nadav Malin

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Each of the eight houses in the Eliakim’s Way project on Martha’s Vineyard sports a 5 kW PV array, which provides most or all of the home’s energy.

Photo: Derrill Bazzy

On June 1, 2010, eight families moved into nearly identical, superinsulated homes on Martha’s Vineyard in Massachusetts. South Mountain Company designed and built the LEED Platinum homes for the Island Housing Trust with the goal of allowing the residents to operate them at net-zero energy, using the 5 kW photovoltaic (PV) arrays on the roofs for power. In case the energy cost savings didn’t provide enough incentive, South Mountain offered a reward to any household that came in at net-zero energy for the first year. Two families achieved this goal, and won their choice of a $400 dollar gift certificate at a local fish market or a one-year membership at the local CSA.

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South Mountain installed equipment to allow submetering of all the major energy systems in the homes, providing an unprecedented window into exactly how the families use energy. A report by South Mountain engineer Marc Rosenbaum, P.E. highlights key insights from this experiment—among them the importance of collecting data monthly. Though variations from the estimated energy use will be greater on a monthly basis than on an annual basis, it allows users to catch meaningful anomalies more quickly. In the case of one family, the data helped discover that a child had turned off an exterior AC disconnect from the PV system during the first month, allowing that family to generate only 279 kWh instead of the 630 kWh that the other seven homes averaged.

In a testament to the efficient construction, water-heating energy exceeded space-heating energy in all but one of the homes. Rosenbaum suggests that a good further investment would be for solar hot water or heat-pump water heaters. The submetering also showed that the biggest loads were the two uses of electric resistance heat: the radiant ceiling panels and the water heaters.

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In the end, two families were able to operate below net-zero energy, while two others were close. One family used a measured 11,635 kWh in one year, nearly 170% of the average 6,873 kWh provided by the solar panels. In all cases, lights and plug loads accounted for about half of total energy use. With that in mind, the report quotes energy consultant Andy Shapiro: “There are no zero-energy houses, only zero-energy families.”

For more information:

The complete report is available at

www.southmountain.com

Comments (3)

1 transportation energy should posted by John Beeson on 09/06/2011 at 09:51 am

I'm guessing this doesn't include each of the families transportation energy? But great comparison for same houses, different lifestyles!

2 Transportation Energy posted by Martin Solomon on 09/06/2011 at 10:34 am

Good point John. Transportation is certainly worth considering, but in this case the houses were all grouped together, so each family had the same opportunity to bike instead of drive. Though it wasn't measured (the energy use was measured using meters at the house), it would be interesting to compare how transportation emissions measure up against the household numbers.

3 Transportation Energy posted by Nadav Malin on 09/13/2011 at 01:33 pm

I did talk about transportation energy with John Abrams, president of South Mountain. He said that the project is about a mile from a local village center, with all basic services. So that is walkable, certainly bikable, but not what urbanites would call "pedestrian friendly".

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August 30, 2011