We have seen how we compare the energy efficiency of household appliances, and now we're going to investigate how houses themselves can be energy efficient in terms of their architecture, insulation, and location.
Many local councils have requirements when building or renovating a house to ensure that it is energy efficient. The Inner West council in Sydney uses the Nationwide Household Energy Rating Scheme (NatHERS) that gives houses a star rating (out of ten) based on the design and construction of the house, which helps people who are buying or renting a home know how expensive their heating and cooling costs will be. The star rating is connected with an energy rating (measured in MJ/m2) and varies based on location, since different parts of Australia require different amounts, and different kinds, of heating and cooling.
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| Here is an example of a star rating and energy rating on a NatHERS certificate for a house in Sydney. The 6.5-star rating means an energy rating around 32 MJ/m2 for this area, which means that, on average, the house requires 32 MJ of energy input per 1 m2 of floor space each year to heat and cool. |
Architects, builders, and homeowners can raise or lower this rating by changing the layout of the house, using different building materials, improving insulation, and ensuring the house is oriented in the best direction to maximise or minimise the heating effect of the sun.
Consider the two houses below in the suburbs of Sydney:
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| Aerial floor plans for two houses and their reported energy ratings |
The first has an energy rating of 51 MJ/m2 and has 10 m2 of floor space, so will require 510 MJ per year to heat and cool. The other house has a much more efficient energy rating of 14 MJ/m2, so even though it is much bigger (with 40 m2 of floor space) it requires about the same amount of energy to head an cool (560 MJ per year).
Discussion
- The larger house in the example above looks like it would require more energy to heat. What kinds of things could be responsible for this difference in rating?
- What could the residents of each house do to reduce their energy consumption without changing their energy rating?
- A house in Darwin receives a 9 star rating if its energy rating is below 164 MJ/m2, while a house in Sydney must have an energy rating below 14 MJ/m2. Why does the star rating system change with location? What other kinds of places would have star ratings like Darwin?
Solar panels
Households with solar panels use less electricity from the grid and so pay less per month, but many homeowners don’t purchase a solar panel because of the upfront cost. So another way local councils encourage energy efficiency is by subsidising solar panels, meaning they share the installation cost with the homeowner. The reduced ongoing cost makes solar panels cheaper for residents in the long run, but how long?
Let's consider a family who is currently investigating whether they should install a solar panel that has an upfront cost of \$2400. They use 5000 kWh per year on average, and pay \$0.30 per kWh. The solar panel produces an average of 2000 kWh per year.
To calculate how much the family has been paying per year we take how many kWh they use per year and multiply that by how much money they pay per kWh:
| 5000 \times \$0.30 | = | \$1500 |
We can make a similar calculation for how much the family would pay if they installed a solar panel, and only had to pay the difference between their typical energy usage and the energy generated by the panel:
| \left(5000 - 2000\right) \times \$0.30 | = | \$900 |
The difference between the yearly costs is \$600, representing the amount of savings per year they would make once the panel is installed. If we divide the cost of the solar panel by how much is saved per year we get the number of years it will take until the solar panels are the cheaper option:
| \frac{\$ 2400}{\$ 600} | = | 4 years |
The graph below shows the cost of the solar option starting higher (due to its initial cost), but notice that it doesn't rise as steeply as the current system (since its ongoing costs are lower):
The point where the two lines intersect on the graph is the point where the solar option becomes the cheaper option - so if they plan on staying in the same house for at least 4 years, buying a solar panel is the best financial decision.
They then speak to their local council, and there they learn that the council offers a subsidy of \$1300 for installing the solar panel - they pay for more than half of the upfront cost. This changes the y-intercept of the solar panel line, and therefore the point of intersection of the two lines:
Discussion
- If the family in the above example planned on moving house within the next 18 months, why would installing a solar panel be a bad idea? Why would it still be a good idea?
- What happens if you install several solar panels and generate more energy than you use? What would this look like on the graph?
- Even if solar panels aren’t the cheapest option, what environmental concerns should we think about when discussing energy efficiency? What other subsidies does your local council provide to address these concerns?