Home energy consumption versus solar PV generation
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?” and How to save energy in your home). They told us what we use our energy to power, 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 might 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.
Please keep in mind that kilowatts (kW) are a measure of instantaneous electricity usage/generation (e.g. right now your system is producing 2kW), whilst kilowatt-hours are a measure of cumulative electricity usage/generation over time (e.g. your system produced 6kWh of solar power today, and your home used 16kWh of power to run its appliances.) When referring to solar PV system capacity, the term kW is usually used–this indicates the ‘peak’ capacity of the panels or system; real-life production will likely be lower, depending on conditions.
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 the solar panel and inverter efficiency. 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 with a net Solar Feed-in Tariff in place (where you are credited for the surplus electricity you export to the grid) the home with the above graph would have no Feed-in Tariff income. They would still, however, be saving the $1.14 in electricity that would otherwise have had to be paid for. Although both the average consumption and generation curves for a given individual home will be slightly different then depicted in this graph, this is more or less a representative 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 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.
As a representative example, if this system was in Queensland, where a a net Solar Feed-in Tariff is in place, its owner would earn: 6.02kWh x 44c/kWh = $2.65 in feed-in tariff income (6.02kWh is the 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 $3.97.
However, if this system is located in NSW, where no state-backed feed-in incentive scheme is currently in place (only voluntary contributions of up to 8c/kWh from electricity retailers), the household would earn 6.02kWh x 8c/kWh = $0.48, plus 8.5kWh x 15.6c/kWh = $1.33, for a grand total savings of $1.81 for the day.
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 slightly 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?“.
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. Even if you live in a state or territory with a net Solar Feed-in Tariff in place, a house with electricity consumption/solar generation as in the graph above will not receive credit on its bill–instead, all of the energy bill savings will come from the avoided need to purchase power from the grid.
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.
Under, for example, the Queensland Solar Bonus Feed-in Tariff scheme, the above household would earn: 4.02kWh x 44c/kWh = $1.77 in feed-in tariff income (4.02kWh is the gross amount of solar energy generated) as well as save: 6.5kWh x 15.6c/kWh = $1.01 in electricity they would otherwise have to pay for (6.5kWh is the amount of generated solar energy your house is consuming) Hence, in Queensland a 3KW system for an average household on a winter day has a total benfit to you of $2.78.
In a state with no government-mandated Solar Feed-in Tariff incentive such as NSW (where some retailers offer an 8c/kWh Solar Buyback rate), this 3kW solar system would earn its owners: 4.02kWh x 8c/kWh = $0.32 in Solar Buyback income (4.02kWh is the surplus amount of solar energy generated and exported to the grid) as well as save: 6.5kWh x 15.6c/kWh = $1.01 in electricity they would otherwise have to pay for, giving a total benefit of $1.33.
How should you use electricity to obtain the most benefit?
Obtaining the maximum benefit from your solar PV system requires a decent understanding of how you use power throughout the day and the feed-in incentives (if any) available to system owner in your state. State-based Solar Feed-in incentives have changes significantly since they began being introduced in 2008-2009. (Click here for an up-to-date list of solar incentives in all of Australia’s states and territories.)
Solar Feed-in Tariffs vs Solar Buyback Schemes
Although NSW once had a gross Solar Feed-in Tariff under the state’s Solar Bonus Scheme, all of the schemes currently operational in Australia (as of 23 May 2012) are net schemes, which pay only for surplus solar power exported to the power grid. The remaining programs come in essentially 3 forms: State government-backed Solar Feed-in Tariffs (SA, Victoria, and Queensland), 1-for-1 Solar Buybacks through electricity retailers (ACT, Tasmania, Northern Territory), and voluntary Solar Buyback schemes, which offer (often nominal) rates for exported solar power that are lower than retail electricity rates (NSW, WA). You can read an overview of these schemes and how to take advantage of them here.
Solar Energy Consultant for Solar Choice Pty Ltd
© 2010 Solar Choice Pty Ltd