Want a Net-Zero Home? Be a Net-Zero Family
By Martin Solomon and Nadav Malin
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.
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.
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.”
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The complete report is available at