CASE STUDY – Mountain Horizons Solar Subdivision

Affordable Net Zero New Construction

The self-powered future has arrived for affordable homes in harsh climates.  The National Affordable Housing Network has developed highly cost-effective net-zero solar powered, homes in Montana,.  “Net zero” refers to an all-electric home that on an annual basis produces as much power from solar as it uses.  The new models are  based on their net-zero ready standard package house plan that the nonprofit has disseminated since the 1990s.

The original energy design was produced in 1980, and began a strong proof of performance, based on simulation and monitoring of performance of the optimized package.

The standard net zero ready package features advanced energy efficiency, heat recovery ventilation, interior heat pump water heating, and passive solar orientation, along with the most efficient yet affordable shelf-ready appliances that each average under 350 KW annual consumption.  The construction sequence is changed to ensure that air tightness features are accurately built into the design ensuring long term performance.  Homes based on this package and  designed in this manner have been built across the US and Canada since the mid-1980s, and we have conducted a large number of detailed studies of performance.   

When whole-house photovoltaic systems became affordable (under $21,000 for a 7 KW system in 2016 and less than $17,000 today) NAHN developed the first four homes to be full net-zero homes in the new Mountain Horizons Solar Subdivision annexed to be part of Whitehall, Montana.  Depending on orientation the solar PV array is located on either one roof façade or one façade on the garage, the solar production is enough to cover all end uses (Space heating, appliances, lighting) for homes with 1,300 SF.  The solar system is part of Northwestern Energy’s net zero program.  The systems range from 27 to 30 panels per system.

The 7 KW system is large enough to power all end uses for most households – the size was based on 10,600 KWH per year, the current national average.  in operation, the home “banks” solar credits during high production times, and uses them as needed.   Most homes bank enough credit in the sunny seasons to cover their energy use during the winter seasons.  If the homeowner wants to buy more power for special uses, this comes from the utility.  

The homes are mostly self-conditioning when it comes to heating and cooling.  The design heat load is 12000 BTUs/hr.  At this low level,  the appliances and people generate more than 55 percent of the space heat needed in winter, and the water heater provides cooling when set to summer or hybrid setting.   In the Whitehall homes, space heat and ventilation used less than 18 percent of total energy consumption in a 7990 heating degree day climate.   Electric heat pump water heater (cooling) used 28 percent of energy consumption. The appliances and lighting used 51 percent of the energy use, with the clothes dryer consuming 10 percent alone. 

Comfort and Safety

The reduced heat loss and warmer indoor surface temperature of the homes creates a supremely comfortable environment, where the interior mass of the sheetrock and insulated slab on grade floor makes for a steady indoor temperature and less body energy loss to cold surfaces.  Heat recovery ventilation keeps the indoor air free of pollutant build up, while reclaiming 70 percent or more of the space heat generated by the home.  The lack of indoor pollutants in building materials makes the homes meet clean air standards needed by those with breathing conditions.  The homes are built with walls made of structural insulated panels (SIP) due to the greater structural strength, storm and water damage resistance of the product.  Combined with other climate-ready features, the homes are ready for storm conditions in the future.

In the future, at least 25 more net-zero homes or net-zero ready homes will be built in the subdivision, where lots are laid out to maximize solar gain.  The cost of the homes is similar to or less than conventional homes, and with federal self help programs or Habitat partnership they are already affordable to Montanans with low or very low incomes.  In the case of Whitehall, the Self Help Program’s net zero homes average under built at a price starting under $190,000, including land.  Based on the current price of energy, the energy package’s cost is recovered in under 10 years.  With diligence, net zero operation with 7 KW could also provide 2500 for transportation uses, enough to drive locally for 8,500 miles.

Solar array on one car garage.

Solar array on one car garage.

SOLAR ARRAY on rear elevation

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