Household GHG emissions and Rooftop Solar

In this article I am going to examine residential energy consumption, its contribution toward the carbon footprint of individual average Australians and using my own household data show the impact of rooftop solar on household greenhouse gas emissions. When we talk about residential energy consumption we are talking about household usage of both electricity and natural gas. Let’s start with electricity.

Australian residential electricity consumption

Data on Australian electricity consumption (1,2) shows a wide variation between households in different parts of the country with low residency dwellings in milder climates averaging around 4000 kWhs/year and larger homes in northern Australia or Tasmania two or three times more than this. There will also be variation among individual residences based on how much gas is used for heating and hot water, the number of household appliances (especially those with high demands like pool pumps and clothes dryers) and the extent to which residents engage in active energy conservative.

Electricity usage itself is only part of the story, what determines GHG emissions and carbon print is how the electricity is generated and hence its carbon intensity. Table 1 shows typical carbon footprints in different parts of Australia.

State	Residents	Electricity UsagekWhs/year	Carbon Intensity T CO2/1000kWhs	Carbon footprintT CO2/person/year
Victoria	2	4835	1.07	2.59
	4	6449	1.07	1.73
NSW	2	5548	0.82	2.27
	4	7799	0.82	1.60
Tasmania	2	8733	0.19	0.83
	4	12475	0.19	0.59
Northern Territory	2	7806	0.64	2.50
	4	10885	0.64	1.74

Source: National Greenhouse and Energy Reporting calculator

This highlights the impact of Tasmanian hydro – despite the fact that Tasmanian residential usage is the highest in the country the resulting carbon footprint is by far the lowest. For the rest of the country carbon emissions from domestic electricity use is more consistent. The data also highlights quite nicely the carbon benefit of higher occupancy dwellings.

A final comment to add some global perspective. Developed countries with the lowest per capita GHG emissions (mostly European nations) are around 5 tonnes CO2/person/year. The average Australian is at half that level based solely on the electricity they use at home. This excludes emissions from transport, food, clothing, entertainment, waste disposal and air travel – clearly we face some challenges getting to world’s best practice.

Impact Of Rooftop Solar

I am in Victoria with a two person household so if my consumption was in line with the published average (1,2) my total household consumption should be around 4,850 kWhs/year.

My actual electricity consumption, including generation from my sixteen x 270 W panel rooftop solar system (a well positioned installation with little overhead shade or obstruction) is listed in the table below.

Transaction	QuantityKwhs	Carbon ImpactTonnes CO2/person/year
Electricity Purchased	1481	0.79
Electricity Sold	4569	(2.44)
Self generated electricity consumed	1410	(0.75)
Total Production	5979	3.20
Residential Electricity Carbon Footprint(Rooftop Impact – Purchases)	4498	(2.41)

Total Consumption	2891	1.54

The data above demonstrates that I have a negative carbon footprint based on my high level of generation and relatively low level of grid purchases (and overall consumption). More specifically, I produce 5979 kWhs/year of zero carbon electricity, all of which offsets high carbon grid electricity by either reducing my own household consumption or through sales back to the grid. I purchase 1481 kWhs/year of high carbon electricity from the grid meaning that my domestic electricity carbon footprint is negative 2.41 tonnes of CO2/year. This compares to a footprint of 1.54 tonnes of CO2/year which I would have had if my electricity usage 2891 kWh/year had all been based on the Victorian power grid – a differential of 3.2 tonnes of CO2/year

It is worth noting that my consumption is well below the average of 4,850 kWhs/year for a two person, Victorian household. Part of this differential is because my house uses gas for cooking, hot water and heating and part may relate to energy saving practices in my house.

Domestic Gas Consumption

Natural gas is widely used in industry as well as for power generation. Residential consumption represents a relatively small proportion of overall usage. Average household gas consumption data is provided in the table below (3)

	NSW	Qld	SA	Tas	Vic	Wa
Homes Connected to Gas (000)	1332	182	427	12	1958	710
% Homes with Gas	42	10	57	5	80	75
Ave household usage (GJ/year)	20	8	17	34	49	15
Gas distribution (km)	26,290	5,760	7,950	710	31,090	14,000
Carbon footprintTonnes CO2/household/year	1.11	0.44	0.94	1.88	2.71	0.83

Note: Carbon intensity for gas assumed to be 5.54 x 10^-2 tonnes CO2/GJ (4)

Clearly gas is used more widely in Victoria than the other states, presumably this is part of the reason why Victoria has lower household electricity usage rates than other states.

My household gas consumption is 26 GJ/year which is well below the average Victoria figure given in the table above. Apparently we must have shorter showers or use our gas heater less frequently than the average Victorian. As shown in the table below, the natural gas contribution to my carbon footprint from my 50% of our household gas use is 0.7 tonnes CO2/person/year.

Combining my electricity and use emissions gives a total of negative 1.71 tonnes of CO2/year a figure 5.4 tonnes below what I have estimated to be the average for a person living in a two person dwelling in Victoria.

Does that mean I am a wonderful person, doing my bit to save the world? Probably not.

	Electricityfootprint	Gasfootprint	Carbon Footprinttonnes CO2/person/year
Average Victorian Household	2.59	1.10 (see note)	3.69
My Household	-2.41	0.70	-1.71
Saving vs Average	5.00	0.40	5.40

Note: I have assumed that the average Victorian house has 2.5 residents

What we can conclude is that where gas is used for heating and cooking it makes a significant contribution to domestic GHG emissions. While gas has lower emissions than coal it is still a fossil fuel and when it burns it produces carbon emissions.

There is, in fact, some variation in published carbon intensity figures for natural gas. I suspect that this is less to do with emissions from combustion chemistry and more to do with assumptions regarding the energy involved in extraction and transport as well estimates about methane leakage from natural gas pipelines. Methane, which is the primary constituent of natural gas, is itself a powerful GHG – much more powerful than CO2. Fortunately it decomposes in the atmosphere (after several decades) so does not have the long term impact of CO2. The carbon footprint for natural gas is highly dependent on how much methane is lost between the wellhead and the point at which it is burnt to generate heat and energy. This is perhaps a topic for a later discussion

Conclusion

So what have we learned? Firstly, that installing rooftop solar can have a significant impact in lowering a household’s GHG emissions. The positive impact of rooftop solar on one’s personal carbon footprint relative to non solar owning Australians is perhaps 5 tonnes CO2/person/year. This differential will, however, reduce over time. As the grid becomes less carbon intensive the “value” of generating zero carbon electricity versus buying off the grid will decline. When the grid reaches zero carbon there will be no value – from a carbon footprint bragging rights perspective – from producing zero carbon electricity. Australian domestic rooftop solar generation, will, however, continue to play an important part in helping the grid get to zero carbon and will also likely continue to provide owners with a financial return in terms of reduced power costs.

The second key point we should take from this analysis is in comparison with Australia’s average footprint of 22.0 tonnes CO2 (equivalent)/person/year the impact of reducing GHG emissions by installing a rooftop solar system (and adopting moderately aggressive energy conservation practices) still leaves plenty of work to do. In round terms, rooftop solar can perhaps reduce one’s footprint from 22 tonnes CO2/year to 17 tonnes/year. What more is needed to get to the OECD average of about 12 tonnes or the level of many European nations which are around 5 tonnes?

Let’s look at GHG emissions beyond household energy usage in the next Journey to Zero Carbon article and examine what needs to happen at both a national and personal level for Australians to reduce their carbon footprint and for the country as a whole to head toward zero carbon.