How Zero-energy Homes Work

Building a zero-energy home isn't simply a matter of throwing in a solar-power system and the highest-efficiency refrigerator on the market. A ZEH is built from the ground up with energy efficiency in mind. It starts with the most basic design and construction elements, focusing on two major areas: reducing energy requirements for the systems that account for most of a home's energy use and increasing the home's built-in energy-generating capacity.

A ZEH incorporates:

And it minimizes energy needs for:

There are lots of tricks builders and architects use to accomplish these goals. For instance, the walls, roof and foundation, and the interaction between the three, can be designed to most efficiently manage water, vapor and heat flow. The roof coating may have a high solar-reflectance value. Eaves and awnings can be strategically placed and sized to block summer sun and allow winter sun. Other design elements may include:

Even with the highest-efficiency design and appliances, a ZEH still uses power, and it's still connected to the electrical grid. So how can it be "zero energy?" That has to do with the installed solar photovoltaic system. The solar panels sometimes provide as much energy as the home uses. And when they don't, they have a payback plan.

A zero-energy home requires significantly less energy than a standard home -- up to 60 percent less. But still, 60 percent less energy needed is still energy needed. To make the home zero-energy, then, it has to produce its own power -- and it has to be clean power, or what's the point?

The solar PV panels accomplish the energy production in a ZEH. (It could also use wind power, but residential wind turbines aren't nearly as common as residential solar systems.) The trick that makes a ZEH different from a regular old solar-powered home is the combination of reduced electricity requirements and increased electricity production. So, whereas a solar home consuming 7 or 10 kWh per square foot per month and producing 2 solar kWh per square foot still has to turn to the grid for a significant portion of its electricity, the numbers for a ZEH are nearly perfectly aligned. A ZEH that needs 4 kWh per square foot has PV panels that can generate, on average, 4 kWh per square foot.

It's "on average" because solar panels rely on weather conditions to operate at their maximum efficiency. In this way, and in terms of actual energy consumption (as opposed to average energy consumption), a zero-energy home is in fact a net-zero energy home.

The idea is that it all balances out.

There are times, like in winter (when days are shorter) or on rainy days, when the solar panels are not going to produce all of the energy the ZEH requires. And there are also times when the home requires more energy, like on especially hot or especially cold days, or when the kids play in the mud and the laundry requirements increase drastically. At these times, the ZEH draws the extra energy it needs from the electrical grid.

And then there are times, when the days are long and the sun is cooperating, and the weather is mild so the heat or air isn't needed as much, when the home requires less energy than the solar panels generate. The solar panels then send their excess energy to the electrical grid, supplying clean energy where it's needed.

At the end of the year, the energy use and energy generation should cancel each other out. Thus the "net-zero energy home."

So, what does all of this energy-efficiency mean for the homeowner? Will a ZEH cost less than a traditional home? In the short run, in the long run, or ever? Or is it all a matter of saving the planet one house at a time?

Why buy one?

Cutting down on carbon emissions is a big deal. The polar bears can attest to that. So can drought-ridden villages in rural areas around the globe. But there are other concerns, too. First, there's the money factor.

Zero-energy homes run the gamut in price. A ZEH built in Frisco, Texas, in 2004 was listed at $1 million; a ZEH constructed in Edmond, Okla., in 2005 had an estimated retail cost of $200,000 [source: Oliver].

Regardless of price range, building a zero-energy home does cost more than building a traditional home of the same size and features. Estimates put the increase at anywhere from 20 percent to 60 percent [sources: ToolBase, Oliver]. It's a pretty big increase, but that can go down considerably when you factor in any available federal and state energy-efficiency rebates.

And then there's the whole $0 energy bill thing. A regular single-family home in the United States pays about $2,200 a year on power [source: EnergyStar]. Reducing that to nothing can save $22,000 over 10 years. If you factor in those government incentives, energy-savings might be able to make up for an increase in purchase price.

There's a wrinkle in the energy-savings estimate, though. A ZEH is only ZE if the people who live in it practice good energy habits. If the homeowners leave the lights on all the time, run the AC at 60 degrees F (16 degrees C) in summer and take daily 30 minute showers, the actual energy use of the home is probably going to exceed its generating capacity. In that case, the energy bill is going to be greater than zero.

In the end, the true energy-efficiency of a zero-energy home depends a lot on the people maintaining it. PV systems require regular checkups to work at their maximum efficiency. Even the highest efficiency air-conditioning setup is going to have to work overtime if people leave the door open in the middle of summer. But if the home is used as prescribed, it could potentially work out to provide substantial benefits -- both to homeowners and to polar bears.

For more information on zero-energy homes and related topics, look over the links on the next page.