What is a Zero Energy House and why build one?

Zero Energy House

What is a Zero Energy House?

A Zero Energy House produces as much energy as the people living in it use. The home of Shay Brazier and Jo Woods generates energy via roof-integrated solar photovoltaic (PV) panels and roof-mounted solar hot water panels.

There’s no energy storage on-site and so when the panels aren’t generating enough energy the Zero Energy House relies on a connection to the electricity grid to import power. When the panels are generating more than is needed in the home, surplus energy is pushed back to the grid. Over the course of a year, the amount of energy exported to the grid cancels out the amount imported, resulting in a net Zero Energy balance.

 

One of the reasons Shay and Jo chose Zero Energy is because it is an explicit and measurable target – a house is either Zero Energy or it is not. Many sustainable, green and eco buildings strive to reduce outside energy needs and associated impacts; a Zero Energy House seeks to eliminate them. 

Shay and Jo’s Zero Energy House has also been designed and built with environmentally conscious and resource-efficient principles in mind. The couple selected products manufactured, used, and disposed of with the environment in mind; implemented waste minimisation measures; and, by achieving Zero Energy, have reduced their carbon emissions.

Why build one?

Through their previous work in renewable energy and green building design, Shay and Jo had an awareness of what could be achieved in their home. They wanted a home that was warm, comfortable and low-cost to run. There were three main reasons that led them to build a Zero Energy House.

1. Financial benefits

Like most people building their own home, Shay and Jo had a key figure in mind – the cost to build. This helped them understand how much they needed to borrow from the bank and how much they would pay back over the mortgage term, around 25 years.

The average Auckland household will spend more than $50,000 on energy over 25 years.

<p>However, they knew there was another important figure – the cost to operate. A BRANZ study found the average Auckland home used 7,970 kWh of electricity a year<sup>1</sup>. At the 2018 rate of 29c per kWh<sup>2</sup> that equates to an annual cost of over $2,000. Consequently, over the 25-year term most people pay back the investment in their house, they’d be spending more than $50,000 on electricity. Add in inflation and changing prices set by private electricity providers and it could conceivably be much more.</p>

Shay and Jo chose to invest in a solar system and other features that would help them achieve Zero Energy (such as a higher performing building envelope) upfront. While this increased their mortgage, it protected them from increasing energy prices and, over that 25-year period, would be cheaper than paying business-as-usual electricity bills.

2. Environmental benefits

<p>While a large amount of New Zealand’s electricity comes from hydropower, in a normal year the country still relies upon non-renewable sources for a quarter of its needs<sup>3</sup>. And hydropower has risks. There have been warnings about low lake levels in 2001, 2003, 2006, 2008, 2012 and 2018. When lake levels are low fossil fuels can be brought in to make up the difference, and with a growing population and energy demand, that scenario could occur more and more often.</p>

New Zealanders also rely on a vast transmission network to move power around the country – more than 70% of Auckland’s energy has to be imported from outside the region. Not only is this network expensive to maintain, it is inefficient. Transmitting electricity results in energy being lost, meaning even more of it has to be produced at source to make sure there’s enough when it reaches the destination.

The Zero Energy House has solar thermal and PV panels that help to reduce:

  • Dependence on the fossil fuels that make up a portion of the country’s electricity supply,
  • Reliance on the right weather conditions (solar panels capture energy even on cloudy days),
  • Energy loss by transmitting energy for a short distance, from the roof to the ground.

3. Health benefits

One of the key project goals for the Zero Energy House was to ensure a warm and dry building with good daylight and fresh air supply that would increase the family’s comfort, happiness and health.

A 2008 study found more than 400,000 New Zealand homes are cold and damp enough to make their inhabitants sick. Not only does this affect the people living inside them, but it costs the country more than $70 million a year in hospital bills and lost productivity.

By creating an indoor environment that is warm and dry throughout, the Zero Energy House eliminates these health risks. And by doing so without requiring heating,  there’s no need to worry about spending money on power to stay warm and dry as energy prices increase.

Finally, by achieving this result without mechanical heating Shay and Jo are avoiding the cold spots common in many homes in which living areas are heated but other areas (including bedrooms) are not. Some 45% of New Zealand homes have visible mould in cold areas like these, which can result in serious health effects.

Achieving Zero Energy

<p>The typical Auckland household purchases 10,660 kWh of energy in a year: 7,970 kWh of electricity and 2,690 kWh from other fuels such as gas or wood<sup>4</sup>. The Zero Energy House aimed to reduce the amount of energy purchased from external suppliers to zero via three design innovations:</p>

  1. Building envelope design. The shape and construction methods used eliminate the need for any heating (or cooling). This normally accounts for around 30% of a standard home’s energy use (and bill).
  2. Solar hot water heating. Most of the hot water needs are provided by a roof mounted solar hot water system. The system used is made in New Zealand and reduces energy needs by a further 25%.
  3. Solar PV panels. Eighty-eight of  these are laid out on the north-facing side of the roof provide the remaining 45% of energy.
Footnotes
  1. Building Research Association New Zealand (BRANZ). (2006). Energy use in New Zealand households: Report on the year 10 analysis for the household energy end-use project (HEEP). p18.
  2. https://www.mbie.govt.nz/assets/87c214b821/sales-based-residential-prices.pdf.
  3. Ministry of Economic Development. (2011). p104. (needs updating.
  4. BRANZ. (2006). p17.

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