Factors affecting solar system production

Question: How much power would you expect your solar system to produce in kilowatt hours (kWh) per day?;Average daily production

See the guide below and then take into account the factors below, which will reduce the systems production.

Average daily production

City 1.0kW system 1.5kW system 2.0kW system 3.0kW system 4.0kW system
Adelaide 4.2kWh 6.3kWh 8.4kWh 12.6kWh 16.8kWh
Alice Springs 5.0kWh 7.5kWh 10.0kWh 15.0kWh 20.0kWh
Brisbane 4.2kWh 6.3kWh 8.4kWh 12.6kWh 16.8kWh
Cairns 4.2kWh 6.3kWh 8.4kWh 12.6kWh 16.8kWh
Canberra 4.3kWh 6.45kWh 8.6kWh 12.9kWh 17.2kWh
Darwin 4.4kWh 6.6kWh 8.8kWh 13.2kWh 17.6kWh
Hobart 3.5kWh 5.25kWh 7.0kWh 10.5kWh 14.0kWh
Melbourne 3.6kWh 5.4kWh 7.2kWh 10.8kWh 14.4kWh
Perth 4.4 kWh 6.6 kWh 8.8 kWh 13.2 kWh 17.6 kWh
Sydney 3.9 kWh 5.85 kWh 7.8 kWh 11.7 kWh 15.6 kWh
Data Source: PV-GC spreadsheet based on the CEC GC Design Guideline
The Rated output is that achieved in perfect laboratory conditions. The CEC design summary software takes these deratings into

Shading

Shading has a large effect on the performance of a solar power system. The time of day matters, if the shading occurs outside the hours of 9am-3pm, generally you should be ok. If it does occur during these hours, ask your installer what the effect will be.

Partial shading of the system also matters as all panels in a section need to run at the same voltage, if one panel is shaded even partially, this will bring down the production of the whole system. There are ways around this such as splitting the array into multiple strings or using micro inverters on the back of panels.

Roof Orientation

In the southern hemisphere, north facing roofs are the best for solar. If your roof isn’t north facing, that’s ok, the loss from facing the system east or west won’t matter too much.

Roof Orientation Production Loss
North no losses
North East/North West 7% loss
East/West 15% loss
South 38% loss

Roof Inclination

The angle of your roof doesn’t massively effect your system’s production. To get the best production, your roof should be pitched at the same degrees as the latitude of your location.

For example, Brisbane’s coordinates are as follows – Latitude 27°25’SLongitude 153° 9′ E.
This means that the best roof pitch is 27 degrees which is about what most roofs are.

If your pitch is different to this, don’t worry, for every 5 degrees difference from this, you lose 1% production. So if the panels were laid flat, you’d only lose about 5% production.

You should at least have about 5 degree tilt so that water can run off and the panel can self clean.

Summer vs Winter Production

Solar systems will always produce more energy in summer than in winter and this is due to the sun being higher in the sky and the days being longer. One interesting point which most people don’t realise is that solar systems don’t like too much heat and will produce around 20% less energy than their peak on a 40 degree day. This is why you may find that your solar system hits its peak production at any one point in time during spring or autumn. Overall though, your system will produce more units of energy during the summer months due to the longer days.

The temperature co-efficient is a guide for how much your panels dislike the heat and also an indicator of quality. This number tells you how much your panels will lose in production for every 1 degree over 25 degrees of surface temperature. The lower this number, the better.

Avg Daily Production = 1kW
(production from table below)
x System Size x 1
– percentage loss from shading
– percentage loss from inclination
– percentage loss from roof direction

Tilting panels

Customers regularly ask us whether or not they should use tilt frames to tilt their panels so that they’re closer to the optimal angle. In 95% of cases, we would suggest using the extra money that you would have had to spend to install the tilt frames and buy some more panels. When you consider that the difference in production for a 25deg roof and a 0deg roof is only 5% production, tilt frames are really not worth the extra hassle.

Other drawbacks of tilt frames:

  • They can catch the wind and can act as a sail and therefore put greater stress on your roof structure.
  • Any increased production is offset by needing to leave spaces between rows of panels so that panels don’t cast shadows over each other.
  • Increased cost for the frames plus increased labour costs.

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