This Habitat for Humanity zero-energy home in Wheatridge, Colo., features solar panels.

Photo courtesy of NREL

Home Energy Production

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?