Heating your home in winter – Firewood versus Sunlight?

I had cause to travel to Trentham in regional Victoria last month to attend a meeting with MASH (More Australian Solar Homes) and talk to locals about solar. Now, Trentham is located at an altitude of around 700 meters, so in April home heating is on most people’s minds.

I had two people ask me the same question – “I have wood heating at home, can a solar system replace my wood heater?” That seemed like a fair enough question and, as I’m no engineer, I asked one of our engineers, Ben to do the investigation. The results are interesting.

Having said that, efficiency IS vitally important, for the reasons detailed below.

Our Findings

We used a baseline required heat output of 9kW and average run-time of 3 hours per day. Of course, a wood fire produces radiant heat as well as convection heat so the comparison is not identical, but it should suffice to answer the question. And other factors such as the quality of the wood, type of fireplace and its maintenance and efficiency can dramatically affect the output. Fortunately, the output from a reverse-cycle air conditioner is a bit more predictable, and we used this electrical appliance as a comparator.

Reverse-Cycle A/C

A reverse-cycle A/C, or heat pump, has the advantage of efficiency, generating almost 4 units of heat for every one unit of energy consumed, so it’s the best form of electrical heating when seeking efficiency. And while you can’t run at night off solar power, we factored 1/3 of the run-time to be from solar panels. We did not take into account the offset of the cost of the A/C system running at night that would be gained from daytime feed in from your solar system, but this would also be a factor. Running your A/C during the day to create a heat load in your house is a great way to ensure greater self-consumption, even if you’re not home during the day.

Wood Fire

 

To generate 9kWh of heat you will require 3kg of wood for one hour, which will cost around $1.33 based on firewood prices of around $450/ton. Over a winter season of 160 days averaging 3 hours per day, this will bring your winter wood heating costs to $639. In my experience most houses use more wood than this each winter, but let’s assume it’s a small, well insulated home for this exercise.

Comparing Efficiency

To generate the same 9kWh of heating, a reverse-cycle A/C or heat pump, will consume 2.4kWh of electricity over the same one hour period (remember 1 unit of power delivers 4 units of heating) and this equates to around $0.62 per hour (on a 26c tariff.) If we can assume 1/3 of this heating will be whilst the sun is out, your solar is running and the power is effectively free, this will bring the cost of heating the same home to around $200 a year, or roughly one third of the heating cost of wood. Not a bad result and while the figures for every home will vary on several factors, the difference is compelling.

And finally, while wood heating is often seen as a clean form of heating, the same scenario above will generate over 3.5 tons of CO2 equivalent over the season compared to around 600kg using the solar system and A/C heating. This is because burning wood releases methane, which is a greenhouse gas 25 times worse than CO2. Not only that, but wood fires can also cause significant pollution due to particulates.

I know from the conversations in Trentham that high quality firewood is getting harder, and more expensive, to obtain. Choosing a high-efficiency heat pump and backing it with solar panels might just prove to be a cheaper, and cleaner option.

Heating Your Home: Which way is cheaper?

Economic analysis of a Split System Reverse Cycle air conditioner compared with a wood fired heater.

Using a split system halves your heating costs!

Could save you $339 a year, or $439 a year if your home has a solar system

Environmental Impact

 

Wood Heaters not as sustainable as you might think.

Burning wood for fire releases methane gas, which is a greenhouse gas 25 times worse for the environment than carbon dioxide.

 

 

 

Using a split system supported by solar power can save 3 tonnes of CO2 every year – the equivalent of planting 134 trees!

 

 

[1] This is an average number, assuming the heater is powered by solar for one third of the time it operates.
[2] Wood burning heaters and your health – Fact sheets (nsw.gov.au)

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