Fancy a life free from power bills? A net zero energy home is one which generates as much energy as it uses.

Energy independence and resilience certainly have appeal. No more shocking electricity bills and ever-increasing power prices. Carbon-free, renewable energy. For the conscientious, the knowledge that your energy footprint is as small as you can make it.

There are just two top-line considerations that sit above the finer details of building a net zero energy house; energy in, and energy out.

During the planning phase of building a home this is best considered in reverse; the less energy needed to run your home, the less will have to be generated.

The average New Zealand home uses about 20kWh per day, but with the right combination of features that can be halved, or even quartered.

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Achieving this level of energy use will slash the cost of energy generation and/or storage so much that the sums can start to look mighty attractive.

Below the top-line basics, considerations are split into four, taken in sequence;

1. Reduce - the reduction of energy use through the use of more efficient technology and the changing of wasteful habits
2. Replace - changing to alternative sources to perform the functions usually powered by electricity.
3. Generation - how to create your own energy.
4. Storage - battery storage for energy produced.

Reduce

The best way to reduce energy use is through passive solar design - the maximisation of the sun's energy.

Large windows on the north side of the home allow the sun a long way in to the building, particularly during winter. That energy can be stored in a 'thermal sink' floor such as polished concrete or tiles, which release the warmth after the sun goes down. This floor will need to be shaded from the midday sun in winter to prevent overheating, which is achieved through a modest eve outside. Conversely, on the other side of the house, it's desirable to have small, or no, windows.

Insulation is key for passive home design, both in windows (double glazing, preferably argon filled) and insulation material - in the roof aim for R-values of 6, 3.2 for timber-framed walls and 1.3 for underfloor.

Replace

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'Replacement' ways to reduce typical energy usage include the substitution of appliances which use electricity for those which use less, or none at all- for example deciding on a pellet fire or wood burner with a wetback water heater instead of a heat pump for space heating.

Heating and hot water account for almost two thirds of a home's energy use.

Solar hot water systems, like the Aquarian one below, are being rapidly eclipsed by heat pump water heaters (HPWH), which use just a third of the energy of a traditional electric hot water cylinder and generally cost less to install. Strictly speaking, they should sit in the above category, but are included here for the purposes of comparison to solar hot water.

Over the course of a year, while solar hot water heating may prove to be cheaper to run during summer, the HPWH comes into its own over winter.

Generation

The darling of small-scale generation is now undoubtedly solar photo voltaics (PV). The pace at which this has happened has been breath-taking, and can be credited to a huge price drop - almost 75% in the last half-dozen years. But, meanwhile, in the background, solar engineers have advanced leaps and bounds in the efficiency of solar panels.

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Until a governing body steps in, power companies are unlikely to pay any more than 5c/kWh for electricity from your solar panels, so that leaves two options; simply install enough panels to cover just your daytime use (which won't result in a zero-energy home, but will still be inexpensive to run); or install enough to cover your entire energy use, and put in a storage system (batteries).

Other micro generation technologies exist, such as turbines - particularly effective in exposed places - and micro hydro generation. These are cheap and effective, but limited to those with a stream running through the property and a decent vertical drop.

Storage

The price drop in battery storage is lagging behind that of solar panels, but is expected to follow suit. In 1997, nearly two thirds of worldwide solar deployment was off grid. Just three years later, more grid-tied systems were installed than off-grid.

However, with the reluctance of power companies to pay much for small scale power generation, subsidies becoming less common and solar and battery solutions dropping in price, conditions are conspiring to bring about a shift back in the other direction. In addition, development of battery technologies now has serious money and technology behind it and is no longer a 'back-of-beyond' technology.

For a home using an average of 20kWh per day, battery storage starts at about $20,000. For a super-efficient home, that cost can be greatly reduced. Add up your power bills and do the maths.

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Off the grid

Waiheke Island resident Dana Darwin is building a zero-energy home. Known as an earthship, it is made predominantly of earth-filled tyres and other recycled materials.

An earthship uses energy from the sun and thermal mass construction to naturally regulate indoor temperature. Orientated to maximise solar gain, the north side of an earthship has a glass greenhouse "airlock" that helps with temperature control, and the home treats wastewater by passing it through botanical cells, provides for food production and supports a row of photovoltaic panels.

This editorial series is made possible with funding from Energy Alternatives. To find out more about energy efficient products visit energyalternatives.co.nz
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