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Solar Generation and Shading

It's widely understood that shading is detrimental to solar power generation. In typical DC string systems, shading on a panel not only reduces the output of the shaded panel, but the entire string of panels that are connected together. Residential systems typically have two strings, so even if only one panel is shaded, it will reduce the generation on half of the system.

What is not well understood is how much shading impacts solar generation. EB Solar uses detailed modelling software to enable us to quantify these losses. We have chosen a house with a chimney as the shading structure as an example of a typical shading scenario.

There are recent advances in PV panel technology which reduce the effect of shading, such as bypass diodes and split cells. The modelling undertaken assumes the PV panels have bypass diodes and split cell and thus the losses due to shading are less than they would be for panels without these technologies. There are other technologies such as Enphase AC systems, in which each panel has its own microinverter, and SolarEdge and Tigo where each panel has an optimiser installed. These will also reduce losses due to shading, as in AC systems the panels are not connected to each other in strings and thus any shaded panel will not affect the output of other panels in the array, while optimisers help to balance the difference in outputs of individual panels in a string.

It is obvious that the chimney, which is located on the northern roof area, will shade any panels installed adjacent to it. As a baseline, we modelled the system generation without the chimney present, which has a yield of 1,555kWh/kWp, i.e. 1,555 kWh per installed kW of solar power. This is very high, and reflects the optimal angle and orientation (North) of the roof.

Adding the simulated chimney, the losses due to shading can be calculated per panel of the array and are shown in the figure below.

In total, the system will produce 13% less energy, with losses mostly in winter, when the shading will be more pronounced. This equates to reduction in annual savings of around $150, and adds a year to the payback period of the system.

In addition to the loss of generation, it is also likely that the shading will reduce the lifespan of the panels directly behind the chimney, as the frequent changes in generation from the shading will put more stress on the electrical components in the shaded PV panels.

In order to work out the optimal roof layout, we modelled moving the 4 panels most shaded by the chimney to the east facing roof area. This resulted in a reduction of 9% when compared with the baseline, and reflects the fact that existing panels are still being shaded to some extent. Adding optimisers to a portion of the panels also decreased losses, but the increased capital cost meant that the payback period of the system was still longer than moving panels to the east. Here is a table summarising the findings:

In summary, the modelling has shown that shading will reduce the output of a solar power system, however the actual reduction is less than what was expected. A substantial shading object like a large chimney will reduce the system generation by around 13%, and it will also likely reduce the lifespan of the shaded panels and thus should still be avoided if possible.

The additional cost of optimisers is not worthwhile purely from a financial viewpoint as it is not recouped in additional savings over the life of the system. However they will reduce the stress on the shaded panels from power mismatches and will thus reduce the likelihood of early panel failure.

Our preferred option still is to adjust the system design to eliminate shading wherever possible, and if shading is not avoidable, then install optimisers on the affected panels, or consider a SolarEdge or Enphase solution.

If you have avoided installing a solar system on your home because you thought shading was too much of an issue, get in contact with us. We can complete the appropriate modelling and help to find solutions so you can go solar in the most effective and efficient way possible.


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