(Part 1 of a two-part series on zero-energy houses)

Most people categorize houses by style, as in Victorian or Spanish, or by floor plan, as in ranch or center hall. The Environmental Protection Agency and the U.S. Department of Energy want you to categorize houses by attribute, as in energy-hog or energy-efficient.

The energy-hog house is not hard to describe. It has single-pane windows, minimal insulation in the walls and attic and 10-to-15-year-old appliances that suck up electricity like there’s no tomorrow.

What constitutes an energy-efficient house, on the other hand, has been evolving.

After years of exhorting home builders and buyers to spring for energy-efficient components, such as high-efficiency furnaces, the Environmental Protection Agency (EPA) launched its whole-house Energy Star program in 1995. The goal has been a house that is 30 percent more energy efficient than one built to the standard of the Model Energy Code that home builders in most states are required to follow.

In 1996, the Department of Energy (DOE) launched its whole-house Building America program. This one ratcheted up the energy efficiency requirement to as much as 50 percent more than the Model Energy Code and challenged builders to achieve it at a modest cost. With clever tradeoffs and prudent substitutions, the dollars saved by making some changes would cover the added cost of others.

After a slow start–change comes slowly in the home building business–both of these programs have taken off. About 10 percent of all new houses built in 2004 will have the Energy Star or the Building America stamp.

Not resting on its laurels, however, DOE is continuing to raise the bar on energy efficiency.

With both the Energy Star and Building America programs, the concern has been the energy required to heat and cool the house and produce hot water. Now DOE is going after all the energy consumed in the house, which includes the 30 to 40 percent consumed by the ever-increasing number of gizmos and appliances that typically grace most households.

DOE’s program is called the Zero Energy House, which DOE defines as one that generates as much energy as it consumes. Such a goal is admittedly ambitious and difficult to achieve, said DOE Assistant Secretary David Garman. In fact, the goal is so elusive that at this juncture a builder can participate in DOE’s program and market a house as a Zero Energy House if the total of its utility bills is 50 percent less than those of a comparably sized standard new house in the same area.

What does a Zero Energy House look like? As with the Energy Star and Building America houses, it looks like every other one on the block with one major difference. A Zero Energy House has photovoltaic cells on the roof that convert sunlight into electricity.

NASA first used photovoltaic cells, generally called PVs, in the 1960s to recharge the batteries on its satellites. Their adaptation to residential and commercial buildings began with the 1973 oil embargo and energy crisis, but PVs at that time were wildly expensive and not widely used. Over the last 30 years, however, their cost has fallen dramatically–from about $40,000 a watt to about $8 to $10 a watt. Nonetheless, a 2.4-kilowatt system for a 2,400-square-foot house is still a pricey proposition–it can cost $20,000 to $24,000.

What does a PV system look like? The most common type has blue or black crystalline cells grouped in panels that are typically 3 feet by 5 feet. A 2.4-kilowatt system requires about 12 to 16 panels. To maximize the PV’s exposure to sunlight, the panels are installed on the south-facing portion of a roof. Because some owners have aesthetic objections to the panels, a thin film that can be attached to a metal roof is also an option, though the film produces only about half as much electricity, said John Thornton of the National Renewable Resource Laboratory (NREL) in Golden, Colo.

In addition to the panels, homeowners also need an inverter to convert the DC current produced by the PVs to AC current, which is compatible with conventional household appliances.

Though you might infer that a PV system would be more appropriate in areas of the United States with lots of sun, such as the Southwest or Florida, there is almost no place in the country that PVs cannot generate a viable amount of electricity for the average household. The cells will generate electricity with cloud cover, rain, and even during a snowstorm, though not as much as they will in bright sun.

The areas in the country where PVs have enjoyed the most success, however, are not related to sunlight hours at all. The critical factors that have captured homeowners’ attention are electricity prices, PV subsidies, and net metering plus time of use rates. Thus far, California has the largest number of residential PVs in the country because it has some of the highest-priced electricity, and some local utilities will subsidize up to 50 percent of the purchase and installation cost of a PV system. In addition, the state offers a tax credit of 7.5 percent. All these inducements reduce the cost of a $20,000 to $24,000 2.4-kilowatt system to about $9,250 to $11,100.

On top of this, some local California utilitiesoffer net metering plus time of use rates to homeowners with PVs who are connected to the utility grid. During daylight hours, the local utility will purchase any excess electricity generated by the PVs and not utilized by the household (the meter literally runs backwards). At night, when the PVs are not working, the household will purchase electricity from the utility. In the course of a year, the homeowners may earn a credit with the utility even when they use more kilowatts than they sell. This seeming contradiction is due to the difference in day and nighttime electricity rates. The daytime rate, especially during the summer peak hours when air conditioners are going full blast, is higher than the nighttime rate when the demand for electricity is vastly reduced.

Another inducement to purchase PVs is the financing. The 50 percent reduction in the utility bills that the Zero Energy Household enjoys increases the household’s monthly net income, and this in turn qualifies it for a higher mortgage.

Of course, if the cost of the PV system were low enough, subsidies and inducements would not be necessary, and many more buyers would be interested. Roland Hulstrom, also of NREL, couched the situation in kilowatt hours (kwh), the unit of measurement used by utility companies when they calculate your monthly charge. Nationwide, the current cost of PV-generated electricity is about 20 cents/kilowatt hour, while grid-generated electricity averages only about 6 to 8 cents/kilowatt hour. When the cost of the PV-generated power is only 10 cents a kilowatt hour, it will be cost competitive with the grid, and sales will take off, Hulstrom said. He estimated that the PV-generated power would fall to that cost point within the next 10 years.

Moving from the micro–individual homeowners–to the bigger picture, how do PVs benefit utilities and induce their subsidies? The PVs do not generate that much electricity, but collectively they generate enough to help a local utility meet its peak summer demand without having to purchase expensive reserves from another source or build a new plant. John Bertolinoof the Sacramento Municipal Utility District (SMUD) said that if half the new houses in his service territory had PVs, SMUD could avoid building peak power plants within its service area every five to 10 years. And that will benefit everyone in the service territory and points beyond.

Unbeknownst to most consumers, fossil-fueled electric power generating plants cause more air pollution than cars, and they are a major source of the greenhouse gases that have led to global warming.

Useful Web Sources:

For more information on photovoltaics, go to the National Renewable Energy Laboratory Web site, www.nrel.gov and click on “solar” on the home page.

For more information on state and local incentives to purchase PVs, see. www.dsireusa.org

For information on renewable energy and solar energy technology, see the U.S. Dept. of Energy’s www.eere.energy.gov/solar

Questions or Queries? Katherine Salant can be contacted at www.katherinesalant.com.

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