Home energy consumption versus solar PV generation

(There have been a number of changes to feed-in tariff policy across the Australian states–most notably that the NSW Solar Bonus scheme has been suspended. Please see our feed-in tariff page to see what solar incentives are on offer in your state!)

You may be wondering, if I install a 1.5kW solar PV system, how much energy will I be feeding back to the grid? This is an important question, especially if your state has a net feed-in tariff. The answer depends on two things – how much energy you produce, and how much you consume.

(Get a free comparison of solar quotes from the installers who operate in your area, anywhere in Australia!)

A previous article on this site looked at the first part of that answer, how much energy your solar cells will produce. Here we learnt that the amount of energy a 1kW system produces varies depending on where it is located in Australia.

The last two articles in this series have explored the second part of that answer; how much energy you consume in your home (How do I use electricity throughout the day “ the load curve and How to save energy in your home). They told us what we use our energy on, when we use it, what uses most of it and also some hints on how to use less. Here we also learnt that the amount of energy the typical home consumes varies depending on where it is located in Australia.

In this article, we stitch those two parts together to find the answer to the question of how much a typical home in Sydney will export to the grid.

Consumption versus generation in Summer

The graph below shows three curves for an average Summer day in Sydney:

1) How much electricity the typical household would consume (the red line)
2) How much electricity a 1.5kW grid connected solar PV system will generate (the solid green line)
3) How much electricity a 3kW grid connected solar PV system will generate (the dashed green line)

How to interpret this graph

The graph below shows the amount of power being used by an average home, and generated by an average  solar PV system at any point in time during an average summer’s day. If the green generation line is higher at any point then the red consumption line, you are generating a surplus of power, and hence feeding that electricity back into the grid. When the green generation line is below the red consumption line you’re still generating, but not enough to meet your households demand, so you will be buying electricity as per normal to make up the difference.

In the discussion below you may get confused by the nomenclature of kW and kWh. They are not the same thing, one is power and one is energy. For an explanation of this please refer to the article; Physics 101 – What’s the difference between power and energy?

Average NSW household in Summer – electricity consumption versus generation

The average production of a solar PV system in Sydney has been calculated using the online performance calculator for a grid connected system; PVwatts. The attentive eye will notice that a 1.5kW system is only producing just a touch over 1kW of power at it’s peak. This is not an error. The average PV system will export only around 75% of it’s rated power to the grid at it’s peak generation due to the variety of losses associated with cleaning, inverting and transforming power from the solar cells to a usable form.

The home electricity consumption curve has been calculated from grid wide electricity consumption data for NSW from the Australian Electricity Market Operator (AEMO).

A 1.5kW system

In the above graph we can see that a 1.5kW system will produce just enough power to very slightly surpass the average household’s demand at 1pm, when the sun is at it’s peak. The rest of the time, the average household uses more than the solar PV cells can produce. In total, the 1.5kW system produces 7.3kWh of energy, compared to total consumption throughout the day of 20.5kWh for the house (for the technically minded, the amount of energy produced is the area under the curve, because energy is the integral of power).

Hence, in this situation virtually no power is exported or fed into the grid by your PV system (except for a negligible amount at 1pm) as it is all consumed by your home. This will save the money that you would otherwise have paid for electricity.

In a state (such as Victoria) with a net with a net feed-in tariff in place (where you are paid only for the SURPLUS electricity you export to the grid) you would have no feed-in tariff income. You would still, however, be saving the $1.14 in electricity you would otherwise have had to pay for. Although both the average consumption and generation curves for Victoria will be slightly different then depicted in this graph, this is more or less a representative example.

(NSW formerly offered a gross feed-in tariff. With a 1.5kW system you would get 7.3kWh x 60c/kWh = $4.38 in feed-in tariff income if you signed up before 27 Oct 2010. Unfortunately, no tariff is on offer for those who sign up since the scheme ended, and no guarantees will be made until April 2012. However, there are still compelling reasons to get a solar system in NSW!)

A 3kW system

The dashed green line shows the electricity generation of a 3kW grid connected solar system. As you can see, this is above the red line for the majority of daylight hours, meaning you will be exporting a good amount of energy to the grid.

In total, the 3kW system produces 14.5kWh of energy, compared to total consumption throughout the day of 20.5kWh for the house. But because it is often producing more at any given time then the household can consume the 3kW system exports a total of 6.02kWh of energy to the grid.

The rate you are paid for your surplus electricity fed into the grid will depend on which state you live in. (Read about state-by-state feed-in tariffs.)

As a representative example, if this system was in Victoria where a a net feed-in tariff is in place, you would earn:

6.02kWh x 60c/kWh = $3.61 in feed-in tariff income (6.02kWh is the net (surplus) amount of solar energy generated and exported to the grid)

as well as save:

8.5kWh x 15.6c/kWh = $1.32 in electricity you would otherwise have to pay for.

Giving a total benefit of $4.93.

(Under NSW’s now defunct gross feed-in tariff scheme, with a 3kW solar system you would have brought you a total benefit of 14.5kWh x 60c/kWh = $8.70 in feed-in tariff income. 14.5kWh is the gross amount of solar energy generated.)

Consumption versus generation in Winter

In winter, the expected solar PV average generation curve is slightly lower then in summer, reflecting the lower intensity of the sun. Note that on a cloudy day generation will be much lower then depicted. In Sydney, where the sun shines almost perenially, this is not such a problem. Melbournians, however, are less lucky! In this graph average household electricity consumption is represented by a blue line. For a discussion as to why it takes this shape, please refer to the article How do I use electricity throughout the day “ the load curve.

Average NSW household in Winter – electricity consumption versus generation

A 1.5kW system

In the above graph we can see that a 1.5kW system will never fully meet an average household’s demand. In total, the 1.5kW system produces 5.3kWh of energy, compared to total consumption throughout the day of 26.7kWh for the house.

Hence, in this situation no power is exported or fed into the grid by your PV system as it is all consumed by your home. In NSW, where a gross feed-in tariff is in place (where you are paid for ALL the solar energy you generate) you would earn:

5.3kWh x 60c/kWh = $3.18 in feed-in tariff income

as well as save:

5.3kWh x 15.6c/kWh = $0.83 in electricity you would otherwise have to pay for

Hence, in NSW a 1.5KW system for an average household on a Winter day has a total benefit to you of $4.01.

In a state like Victoria, however, where a net feed-in tariff is in place (where you are paid only for the SURPLUS electricity you export to the grid) you would have no feed-in tariff income. You would still, however, be saving the $0.83 in electricity you would otherwise have had to pay for. Although both the average consumption and generation curves for Victoria will be quiet different in winter due to the prevalence of gas heating, this is an illustrative example.

A 3kW system

The dashed green line shows the electricity generation of a 3kW grid connected solar system. As you can see, this is above the blue line for the majority of daylight hours, meaning you will be exporting a good amount of energy to the grid.

In total, the 3kW system produces 10.5kWh of energy, compared to total consumption throughout the day of 26.7kWh for the house. But because it is often producing more at any given time then the household can consume the 3kW system exports a total of 4.02kWh of energy to the grid.

In NSW, where a gross feed-in tariff is in place you would earn:

10.5kWh x 60c/kWh = $6.30 in feed-in tariff income (10.5kWh is the gross amount of solar energy generated)

as well as save:

6.5kWh x 15.6c/kWh = $1.01 in electricity you would otherwise have to pay for (6.5kWh is the amount of generated solar energy your house is consuming)

Hence, in NSW a 3KW system for an average household on a Winter day has a total benfit to you of $7.31.

As a representative example, if this system was in Victoria where a net feed-in tariff is in place, you would earn:

4.02kWh x 60c/kWh = $2.41 in feed-in tariff income (4.02kWh is the net (surpluss) amount of solar energy generated and exported to the grid)

as well as save:

6.5kWh x 15.6c/kWh = $1.01 in electricity you would otherwise have to pay for

Giving a total benefit of $3.42.

How to boost your solar generation above consumption

There are two ways you can boost your generation above consumption, and hence export more energy to the grid.

One way is to first lower your consumption of energy. The previous article, how to save energy in your home, provides some practical hints on how to do this. In particular, for the purposes of the net feed-in tariff it is advantageous to focus your efforts on saving energy during daylight hours when your PV cells will be generating electricity. Another strategy would be to shift when you use appliances, for instance, run the dishwasher at night instead of during the day. Although maximising your solar feed-in tariff, this does not have any positive environmental outcome. Hence, the best action you can take as a environmentally conscious citizen is to try and reduce your energy consumption overall and become more energy efficient. This way you’ll earn more from a net feed-in tariff, pay less in energy bills and use less brown electricity.

The other way to maximise the difference between generation and consumption is to simply install more solar PV generation capacity. This has been demonstrated in the above examples by the observation of the differences between a 1.5kW and 3kW solar PV system. Different states have different capacity limits for installed PV generation (read about the status of feed-in tariff schemes in each state) so make sure you’re within these. If you can’t install a bigger system because you’re near the capacity limit or have run out of available roof space, another way to further maximise your generation is to install a solar tracker.

Kobad Bhavnagri
Solar Energy Consultant
Solar Choice Pty Ltd

© 2010 Solar Choice Pty Ltd