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Can you Convert a House Star Rating to Dollars?

What’s a Star Rating worth? How can you estimate the cost of electricity for heating & cooling your home from a House Star Rating? Is it even possible? It is, but it requires a little work. You’ll need to find a few extra pieces of information. First, here’s a little background knowledge.

What's a Star Rating worth?

A house’s Star Rating is calculated from the total annual energy required for the house to stay at a comfortable temperature all year long. This is done by a computer simulating the building in one of a few different software packages that calculates everything thermally relevant to a house. This computer simulation is not just for the building itself, but includes extra details like the geographic location for sun angles, the local climate, assumed average occupancy of individual rooms, the heat given off by appliances in different rooms, and importantly, the thermostat settings for the cooling & heating system.

The software’s Cooling thermostat (the hottest a room can reach before air-conditioning cooling is applied) varies across Australia & ranges from 22.5°C to 28°C. The software then increases this maximum value proportionally in each room depending on the amount of airflow in the room – whether from windows, ceiling fans or even a stack-based passive ventilation system – because people can tolerate a higher temperature if there’s some airflow cooling them down. The Heating thermostat (the coldest a room can reach before heating is applied) is much more rudimentary – it’s set according to the type of room & the time of day, but it’s normally around 20°C for living spaces during daytime.

The simulation software uses a full fluid dynamics engine to calculate how much energy is used to Heat & Cool the building for every square metre of floor area, for every individual room, for every hour of every day, for the entire year (phew!). The end figures are given in raw MegaJoules of heating & cooling energy required per square metre, or MJ/m². The total of these two numbers is used to calculate the building’s final Star Rating. A higher Star Rating will have a lower total Cooling + Heating value (it uses less energy).

Keep in mind that this is just an estimate of your predicted heating & cooling costs, assuming an “average” lifestyle. Everyone lives differently so it won’t be a 100% match but it’s a good ballpark figure. It also doesn’t include any other appliance running costs, just your air-conditioner/heater.

Now that that’s out of the way, here’s how to convert a Star Rating to Dollars:

  1. Find the house’s separate Cooling MJ/m² and Heating MJ/m² values (not the combined total) from the Energy Efficiency Assessment Report, or if it’s not on the report ask the Energy Assessor for these numbers from their calculations.
  2. Find or calculate the “conditioned” floor area (this should also be on the Energy Efficiency Assessment Report). The conditioned floor area is the floor area that is normally air-conditioned. This number is the floor area of all “habitable” rooms (excludes Garages, Verandas or Patios, Laundries & Bathrooms, but including Ensuites attached to Bedrooms). The conditioned floor area will be different to the total floor area on the plans since the total floor area includes these rooms that aren’t “conditioned”.
  3. Multiply the Cooling MJ/m² by the conditioned floor area then divide by 3.6 to get the kilowatt-hours (kWh) of Cooling energy. Likewise, multiply the Heating MJ/m² by the conditioned floor area then divide by 3.6 to get the kWh of Heating energy. These are the raw “displaced” energy numbers – they represent just the amount of raw cooling/heating energy displaced, not the amount of energy used by an air-conditioner to actually cool or heat your home. We still need to apply an Energy Efficiency Ratio to get the actual values.
  4. This next part depends on how you heat & cool your home.
    • If you have a reverse-cycle air-conditioning (a/c) unit, you need to find the Cooling Energy Efficiency Ratio (EER) and the Heating Coefficient of Performance (COP). These are different numbers because reverse-cycle heating is more efficient than cooling. You can find these numbers from a few different sources:
      • The identification sticker on the a/c unit itself or the “Appliance Star Rating” sticker. However the Appliance Star Rating stickers in particular are often removed on installation, damaged or too faded to read
      • The manufacturer’s brochure that came with the a/c unit, or on the manufacturer’s website
      • Go to and search for your a/c system’s make & model. if you have a split system air-conditioner make sure that you match up both the indoor & the outdoor model numbers, as they can be different
      • If you can’t find your system’s details but you can remember its Star Rating, you can use some generic numbers. A 0 Star a/c unit has a cooling EER of 2.25, and it goes up by 0.5 for each extra Star. So for a 4 Star unit, the EER would be 2.25 + (4 × 0.5) = 4.25. The same values apply for heating COP too

      If you don’t have an a/c unit or you can’t find it’s Star Rating, then use an EER of 2.9 (1.3 Stars) for Cooling & a COP of 3.2 (1.9 Stars) for Heating.

    • If a heater is not reverse-cycle (e.g. a radiator, oil-filled convection heater or a portable electric fan heater) then the COP equals 1.0.
    • If you have another type of cooling or heating system (gas/wood/kerosene heater, evaporative cooling, stack-based ventilation, passive solar cooling/heating, geothermal, etc) then the Star Rating can’t be directly converted to dollars. You will still need to know your system’s EER/COP values and the fuel cost, but you’ll also need to know the estimated number of hours spent cooling or heating. This is not easily available as it isn’t provided as a standard number on Energy Efficiency Assessment Reports. However it can be calculated by a skilled thermal assessor – contact us if you would like us to simulate your house & provide you with these numbers.
    • Divide the displaced Cooling kWh (from step 3) by the Cooling EER, divide the displaced Heating kWh by the Heating COP, then add these two numbers together. This gives the annual electricity consumption in kWh.
    • Find a recent electricity bill & find your Tariff cost in c/kWh. If you can’t find a recent bill, the standard domestic electricity Tariff 11 in Queensland is around $0.30/kWh (June 2014 prices). Multiply this cost by the annual electricity consumption. This is your estimated annual heating & cooling bill. And there we have it, phew!

Here’s an example house: imagine a 5-Star house with a conditioned floor area of 230m². This house has an Annual Cooling Energy of 29MJ/m² and an annual Heating energy of 26MJ/m². The Air Conditioner is Cooling-only with a 4 Star Rating & they have some portable electric fan heaters for winter.

The Air Conditioner’s Cooling EER is 2.25 + (4 × 0.5) = 4.25
The Fan Heater’s Heating COP is 1.0
The Cooling displaced kWh is 29 × 230 / 3.6 = 1853kWh
The Heating displaced kWh is 26 × 230 / 3.6 = 1661kWh
The Cooling electric kWh is 1853 / 4.25 = 436kWh
The Heating electric kWh is 1661 / 1.0 = 1661kWh
The total electric kWh is 436 + 1661 = 2097kWh
The total electric cost is 2097 × $0.30 = $629 per year

Now let’s imagine a 6-Star house with a conditioned floor area of 180m². This house has an Annual Cooling Energy of 18MJ/m² & an annual Heating energy of 24MJ/m². The Air Conditioner has a 5 Star Rating for Cooling & a 5½ Star Rating for Heating.

The Air Conditioner’s Cooling EER is 2.25 + (5 × 0.5) = 4.75
The Air Conditioner’s Heating COP is 2.25 + (5.5 × 0.5) = 5.0
The Cooling displaced kWh is 18 × 180 / 3.6 = 900kWh
The Heating displaced kWh is 24 × 180 / 3.6 = 1200kWh
The Cooling electric kWh is 900 / 4.75 = 189kWh
The Heating electric kWh is 1200 / 5.0 = 240kWh
The total electric kWh is 189 + 240 = 429kWh
The total electric cost is 429 × $0.30 = $129 per year

The first house costs nearly 5 times as much to keep comfortable – and at first glance, there doesn’t seem to be any huge differences between the two houses!


If you’re building a new house, here’s some things you can do to save money on the heating & cooling bills:

  • Try to avoid building a larger house if you don’t need the extra space. It sounds simple, but every extra square metre of floor area is another square metre that has to be conditioned (this will also save you lots of money on construction costs!)
  • Use smaller windows where possible (but make sure you still have enough natural ventilation; this will save money on construction costs too since walls are usually cheaper than windows)
  • If you must use large glass areas, consider low-e glass or even double-glazing. In some cases it is difficult to achieve a high Star Rating without using high-performance glazing
  • Build dividing doors into the house wherever practicable (especially stairwells) so unused areas can be closed off when people aren’t occupying them
  • Apply weather seals to all external doors & windows. Remember that a weather seal only on the bottom of a door may stop the rain, but it won’t stop air leaking through the remaining 85% in the other three edges!
  • Make sure the insulation is of a suitable thickness & is properly installed. Avoid downlights or use downlight covers so the ceiling can still be properly insulated. Small holes may look insignificant but they quickly add up – imagine how well a boat would float if it had many small holes in its sides
  • Install thick drapes with a box pelmet – this not only helps block external sounds for a quiet night’s sleep, but it nearly triples the average window’s effective insulation value

If your house has been already built, here’s some things you can do to lower your home’s heating & cooling bills:

  • If the temperature outside is cold enough or hot enough that you’re shutting the windows & turning on the heater or air-conditioner, shut all the doors in the house except for the rooms you’re living in to avoid heating or cooling unnecessary rooms
  • Avoid using portable air-conditioners or heaters. These normally aren’t very efficient & consume a lot of electricity for a small amount of air-conditioning or heating, sometimes as much as six times as a similar capacity fixed reverse-cycle air-conditioner/heater
  • Try to dress appropriately for the weather – this doesn’t mean you have to be freezing in your own home, but putting on a warmer jumper & lowering your heating thermostat by just two degrees can save you up to 30% on your heating bill
  • Turn off your appliances at the wall when not in use, especially in summer. A lot of appliances still consume electricity when on standby, and every extra kilowatt-hour of heat being generated in the house is another extra kilowatt-hour of heat the air-conditioner will have to remove
  • If you don’t have ceiling or roof insulation, consider installing some. R3.0 is a good price/performance value for retrofitting to an uninsulated home
  • Consider swapping your existing curtains or blinds to thick drapes with a box pelmet – this not only helps block external sounds for a quiet night’s sleep, but it nearly triples the average window’s effective insulation value

If you’re interested in a full thermal simulation of your house to find some suggested renovation improvements, making your new building extra-efficient or even just some help making your dream design comply with the building code, contact us for a fee proposal.

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