Performance of your solar PV system
Photovoltaic (PV) power systems convert sunlight into electricity by a process of turning the photons in sunlight into electricity. The modules produce DC electricity which is then converted to AC electricity (the type that is used in your home) by a device called an inverter. The electricity can be used in your home or exported to the grid.
If your system is producing more electricity than is being used in the household, it will be exported to the grid. This means the electricity feeds back through your meter and joins the electricity network: this is the electricity that you may receive a credit for on your electricity bill.
If the electricity usage in your home is greater than what the system is producing then it will source electricity from your solar power system first and import the remainder from the grid. When this is the case, you will not receive a credit for any electricity produced, however you will not have to purchase it from your retailer either.
Like the usage within your home, solar power system output varies, and there are a number of reasons for this and these are covered in the following section.
Performance
A solar power system is categorized according to the amount of energy it is likely to produce for a given array size, rather than the size of the array alone. For example a 1.5kW system will not produce 1.5kW of power at all times. Rather you can expect it to produce on average 6.49kWh of energy per day throughout the entire year, and you can expect that the system will produce a varying amount of power throughout the day.
Why does it vary?
The testing of a solar module, like many power generators, is done under ideal laboratory conditions which do not vary, and this is to determine what is known as the peak rating of the module. This determines what the module is capable of producing under a standard set of conditions. However conditions like sunlight, temperature and shading change and this can affect the ultimate performance of the module.
What has an effect on output?
There are many factors that affect the output of a system such as the module’s operating temperature, the effects of dust, power loss in cables as well as the losses in converting the DC power from the solar module into AC power. Factors also include the array orientation with respect to ‘north’ and the tilt of the array.
Solar radiation
The amount of energy landing on your modules will vary throughout the year, with the maximum amount of energy landing in the summer, and less than half that likely to land in the winter.
Operating temperature
The power rating of a solar module is determined at an ideal 25°C cell temperature. The typical operating temperature of a solar module is actually about 30°C above the ambient cell temperature. When cells get hotter their production reduces by around 0.5% per degree above 25°C. On the other hand, when the modules get cooler, their production increases by the same measure. Although in winter and the cooler months there is far less incoming solar radiation, the temperature is also lower, and the output is increased because of this – this is something to watch for in cooler months when the sky is very clear.
Orientation and tilt
Orientation of the array will affect the output of the system. However, it is not as critical as many people believe. For maximum energy output ideally the array needs to face due north and be at an angle of around 24-32° above the horizontal. The chart shows the effect of an array in Perth that is not ideally orientated. The example point is shown for a roof slope of 22° facing north-west. In this case the energy output will still be between 94% and 96% of an ideal system.
Even a system that is pointing due east or west will reduce in output by only around 15% from ideal. Generally, the further a roof is oriented from north, the more appropriate a flatter pitched roof becomes with respect to output. Because tilt frames are more expensive than normal pitched roof framing, Sungrid does not recommend using them unless you have an extremely flat roof, usually less than 10 degrees from horizontal.
Shading
Shading has a major effect on a solar power system and as such should be avoided where possible, although a small amount of shading is unavoidable in the majority of cases.
Most arrays will be shaded for a brief period in the morning and also in the afternoon prior to the sun going down.
How much energy will I get?
All of the above factors mean that the output of your system will vary all of the time. However except in bad weather, your system should consistently produce a certain amount of energy per day throughout the year. The table below shows what you can expect as an annual average, but this will vary from month to month and according to the installation configuration of your array.
| Energy Production in kWhr (units) | ||||||
|---|---|---|---|---|---|---|
|
1.5kWp PV Array
|
2.1kWp PV Array
|
|||||
|
Annual
|
Monthly
|
Daily
|
Annual
|
Monthly
|
Daily
|
|
| Adelaide |
2170
|
181
|
5.95
|
3030
|
253
|
8.30
|
| Alice Springs |
2240
|
187
|
6.14
|
3140
|
262
|
8.60
|
| Brisbane |
2100
|
175
|
5.75
|
2940
|
245
|
8.05
|
| Cairns |
2280
|
190
|
6.25
|
2300
|
267
|
8.77
|
| Canberra |
2180
|
182
|
5.97
|
3050
|
254
|
8.36
|
| Darwin |
2220
|
185
|
6.08
|
3110
|
259
|
8.52
|
| Melbourne |
1930
|
161
|
5.29
|
2710
|
226
|
7.42
|
| Perth |
2370
|
198
|
6.49
|
3320
|
277
|
9.10
|
| Sydney |
2020
|
168
|
5.53
|
2830
|
236
|
7.75
|
Explanation of power and energy
There are two important terms that people often get confused with when talking about electricity. The first is the power rating and the second is the amount of energy produced.
Power is measured in Watts (W) or kilowatts (kW=1000W) and this is the number that you would expect to see on the display that tells you how the system is performing at a single point in time. This is the number that you may see vary from day to day, and even from moment to moment as the incoming energy, module temperature and weather conditions change. Power is the measure of the rate at which energy flows.
Energy (an amount of power multiplied by a period of time) is measured in kilo Watt hours (kWh, or “units” on your electricity bill) and it describes how much energy your power system has produced, or how much you have used on your electricity bill.