“Net zero and near net zero homes take the goals of sustainable building one step further. But just what is a zero energy house?

Not too long ago, a house that used 60 percent less energy than one built to code was called a near zero house. That was quite an accomplishment. A house that was this energy efficient could be constructed mostly with conventional materials and techniques but with more attention to detail, such as air sealing and insulation.

Today, building standards are getting tougher, and there are many labels for super-efficiency. There are zero energy homes,  net zero homes, carbon neutral homes, and off-the-grid homes. What are we talking about here?

The simple definition is that a net zero energy building produces as much energy as it uses on an annual basis. This includes energy for heating, cooling and all the devices that plug into the wall. Net zero houses are typically connected to a local electric utility. They use the grid for storing excess electricity generated by photovoltaic panels or a wind turbine, banking electricity at times of plenty and drawing on the surplus when production falls.

A house in a cold climate might need more every than it makes during winter but then makes up for it in summer when demand is lower and the photovoltaic system running at full tilt. The opposite may be true in the south, where high humidity in the summer requires more electricity for air-conditioning during peak months. But on average, zero energy houses produce enough energy to offset the high-load months.

Most grid-tied homes are built where the local utility offers net metering. That means the utility will buy electricity at the same price it charges, but usually only until the net is zero. If, houses produce more than that, the utility may buy it back, but at a very low price, which makes excess energy production very expensive. In Germany, the government has imposed rates on utilities, forcing them to pay roughly 50 cents per kilowatt hour as an incentive to building owners to produce more electricity than they use.

Off-the-grid houses must provide all the electrical energy its occupants need, summer and winter. Other than small battery banks, there is no place to store energy. The house is truly self-sufficient….Most often what makes house self-reliant are changes in life-style for the families that live in them. Electricity goes on a budget. There is a fixed amount of energy available for any given day. If someone wants to take a hot shower, it has to be on a day with plenty of sun. If you want toast in the morning, maybe you can’t use hairdryer. Most American aren’t willing  to adjust their lifestyles that radically.

A carbon-neutral home uses a different metric to determine how to get to zero. More than just zero energy, it must be zero carbon emissions all the way back to the power plant or manufacturing facility that made the building products in the first place. On average, getting electricity from a power plant to a house is at best 30 percent efficient. From a carbon-neutral standpoint, the electricity used from the grid has to be repaid with three times more site-generated electricity to break even. The same holds true for building materials. If the marble tile in the foyer is from Italy, the energy produced at the house has to be sufficient to make up for the embodied energy from extraction and transportation of the marble. The utility buy-back policy also dictates the financial context for this approach. Adherents to carbon-neutral houses are insistent on using only local materials and simple solutions to getting to zero energy. The more complex the house the more diverse the sources of the materials and the more energy needs to be produced.

Size also comes to into the net zero discussion. Some say a 10,000-sq.ft. home can never be sustainable. It is just too big and energy and materials intensive.

Building Houses a New Way

There is a way to build homes that provide financial security for its owners. By minimizing utility bills or even creating the potential for the home to make money by selling energy to the utility at some point in the future, zero energy homes offer a new direct for housing. It is a win for home owner, for the planet and for a new generation of builders who will be able to construct houses that will be durable and will meet future energy challenges.”

 

Recourses

  1. Towards a zero energy home. A complete guide to energy self-sufficiency at home, David Johnston, Scott Gibson.