On a shady, tree-lined street in Willow Glen—a small, progressive community within San Jose, California—sits a home that looks fairly innocuous from the outside. Stone pavers lead to the front porch of the two-story, 3,200-square-foot new build that rests on a solid bass rock foundation and is dressed with cut veneer stone and a sage green façade. It has attractive architectural features, such as a gable roof, bamboo flooring, and timber front porch columns made of century-old Douglas fir salvaged from San Jose’s recently deconstructed Del Monte cannery, but it may not necessarily stop passersby. However, the house was recently purchased for $1.8 million, which is about 10 percent more than area homes of similar size and style.

What distinguishes the home at 1820 Cottle Avenue is what can’t be seen from the street, or even physically seen inside. This is a net-zero house. At roughly 10,000 kilowatt-hours each year, it generates enough energy to power itself for free, offload 2,000 kilowatt-hours of surplus electricity to the local utility company at 28 cents per hour during peak daytime hours, and recover 6,000 net kilowatt-hours of electricity to power the Nissan Leaf parked in its garage for up to 20,000 miles per year. The homeowners’ utility bill is about $15 a month—roughly 3 percent of what its neighbors might expect to pay.

Designed and built by Allen Gilliland, founder and owner of One Sky Homes, the home is LEED Platinum, Passive House certified, HERS certified, EPA Indoor Air Quality certified and is also the first home recognized by the California Energy Commission for achieving the zero net energy standard, which will be required for all homes in California by 2020, thanks to a bill signed into law by former governor Arnold Schwarzenegger.

Energy Use in the Cottle Home

Cooking 600-700 kWh, 6-7%
Lighting 800 kWh, 8%
Hot water 1,500 kWh, 15%
Electronics 2,000 kWh, 20%
HVAC 2,000 kWh, 20%
Appliances 3,000 kWh, 30%


Although the roof design, sun orientation, solar paneling system, window glazing, and wall insulation are each important to the home’s efficiency, nothing about the materials or building assembly here is particularly new or jaw-dropping. Gilliland says the project’s real challenge—aside from convincing prospective home buyers that the $100,000 in hard upgrades would pay long-term dividends in energy savings, comfort, and air quality—was to develop accurate models for predicting and measuring thermal flow.

Gilliland found an early partner in Davis Energy Group, an engineering team affiliated with the U.S. Department of Energy’s Building American program, and recruited the support of electricians from Pacific Gas and Electric Company (PG&E) to wire the house in a way that would measure 48 unique data log-in points and aggregate energy usage types—HVAC, cooking, hot water heating, electronics, lighting—into clearly quantifiable units.

“Nobody had written a book about it; I didn’t have a recipe,” Gilliland says. “One of the things we discovered right away was the Passive House standard. We thought immediately, ‘That’s the answer right there.’ I love the concept. The essence is thermal energy power—planning for natural or passive thermal energy flows through a building.”


The home is a marvelously fluid system of heat, water, and air exchange. On the energy-production side, harnessing the power of the sun is important. Mounted on top of the southern-facing gable roof are 28 three-foot by five-foot Evergreen photovoltaic solar panels capable of generating 10,000 kilowatt-hours of renewable electricity each year. While the panels are generating electricity, a three-panel solar thermal system heats water that is circulated from a storage tank in the garage to produce 70 to 75 percent of the energy needed for the home’s showers and laundry. Recycled wastewater from these hot-water systems is then treated and filtered by a Flotender system before being used to irrigate the trees and shrubs in the backyard.

With its narrower façades facing east and west, the home’s orientation takes advantage of the seasonal sun angle variation that occurs along the northern and southern exposure. At the winter equinox, when the sun’s angle is approximately 30 degrees at 37 degrees latitude, more light streams through the windows to provide heat for the home. In the summer, when the sun’s angle is 76 degrees, little sunlight reaches the windows. The eave and projection of the roofline, along with the glazing features of eight-foot-tall triple-paned German patio doors and windows, also help regulate light entry. The net result, Gilliland says, is 25 million BTUs of energy production.



If optimizing energy production is important to achieve the net-zero energy standard, so too is keeping that energy contained. A thin layer of densely packed cellulose in the wall assembly improves the insulation of the wooden frame. Behind two-foot by six-foot wooden studs—the weak point in the building envelope where insulation is limited—a one-inch-thick skin of polystyrene eliminates thermal bridging. To keep the home dry and well-drained, Keene Building Systems was contracted to install stucco cladding, a Tyvec weather barrier, and a quarter-inch air gap adapted to the coastal climate.

In all of the home’s systems, efficiency is the watchword. A passive heat exchange core, similar to a radiator, keeps the temperature consistent throughout the home while ensuring an 85 percent thermal recovery rate. The home can be heated for 45 cents a day with roughly the same amount of power used for a 1,600-watt hairdryer. But with the exception of a specialized market of Silicon Valley technology professionals, the real estate market has yet to catch on. “A typical profile of our customer is a technology guy, an engineer at Apple or Skype or something,” Gilliland says. “I can’t imagine any of our customers who aren’t involved in technology in some way. This is for early adopters, people who understand the technology and have a value set.”