Heating in Hobart for less than $10 a year?

Or the long winded title: Heating in Hobart for less than $10 a year? Did we overcapitalize/ did we freeze? How many stars (rating) is enough? How much insulation is too much? Where does all the power go? And other ramblings.

This post is part of my Sustainable House Day submission.

Below are the 34 times we’ve used the heat pump since we moved in. Once it got down to 16 degrees in the living room we generally put the heater on for an hour or two. On its lowest setting the heater would increase the room temperature by 2 degrees per hour, and the thermostat is set cut out @ 20 degrees.

Taking advantage of tariff 93 to heat in the off peak period, we spent $3 during the off -peak periods and $4 during the peak period.

heat pump usage for the year – mainly on winter days when there’s been no sunshine for more than a few days
days of heat pump usage over winter

What did it cost us to achieve this level of thermal comfort? Below is my honest best guess of the additional costs above the level of a 6-star house based on the detailed quote our wonderful builders provided:

fully insulated concrete slab10,400
140mm wide stud framing640
extra roof framing2,700
suspended ceiling1,000
thicker wall & roof insulation840
additional air tightness6,700
triple glazed doors & windows instead of double glazed6,600
heat recovery/ ventilation system11,500
extra cost to get close to a 10-star rating

This roughly represents a 10% premium over a 6-star house.

According to the federal government NatHERS “house star bands & energy efficiency rating” table for Hobart climate zone 26 our combined heating/ cooling load was 1.1MJ/m2.PA which puts the house above a 9-star rating. It’s probably not that good as we only heat the living room between 16 & 20C, others may not be prepared to put on a warmer clothes rather than turn the heater on. Still it’s indicative I guess. Andrew comfortably wears shorts inside all year round.

So how much would a 6-star house cost to heat to similar levels of comfort? hmmm, how long is a piece of string?

Our electricity supplier Aurora provide a graph of typical electricity usage for various households. My estimates are based on the following summer & winter charts:

From the same NatHERS table a house is classed as having a 6-star rating if it uses 155MJ/m2.PA. For a 100m2 floor area this would be 4700kWh of electricity per year. Lets be generous & assume this is all on the low cost tariff 41 of $0.17/kWh, so the total yearly power bill will be $800. I haven’t taken into account whether this is 400% heat pump power or 100% or worse plain electric element power, but the figure sounds in the ballpark, and maybe under.

So finally, $40,000 / $800 = 50 year payback period. hmmm, at that rate you’d have to say the extra cost of achieving a thermally comfortable house with virtually zero heating/ cooling bill wasn’t worth it. Of course it’s only in the last few years that houses have had to meet a 6-star rating. You can draw your own conclusions.

Below is our daily total electricity usage:

about 3kWh/person/day (around $0.60)

and extracted from that our hot water heating, which Australia-wide by some estimates accounts for 30% of all power generated:

mostly of this electricity is from solar or worst case $0.15/kWh off-peak

Here’s a breakdown of our major energy consumers:

a lot of this power comes from solar

10% just for the kettle you ask??? That’s a lot of cups of tea & coffee? Well… it only appears a lot relative to other low power usage loads. 0.5kWh/day for the kettle is roughly the equivalent of boiling 1.5 litres on 4 occasions.

Major energy saving measures since moving:

  • a new energy efficient fridge
  • washing machine water is only heated to 20C
  • HRV unit recovers otherwise lost heat during ventilation
  • smallest & nearly most efficient heat pump that could be found
  • heat pump hot water uses 1/4 the energy of a typical vitreous enamel system
  • 5kW solar PV system

I’ll present our solar Photo Voltaic (PV) experience & thoughts in a future post but below is our imported energy from the grid (red) and exported energy into the grid (green). Even at the depths of winter with our PV array pointing SW we were energy neutral:


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