Shading on solar panels and how to defeat it without breaking the bank

The picture above shows a home in the Perth hills area with the Nearmap image taken at 1pm in Winter. The picture below is the same home with the picture taken at 1pm in Summer.

 

 

Living in an area with a lot of trees poses problems when designing a solar installation, but the most obvious thing that we can all ask about this solar install, is... "Why didn't they install the panels further to the Eastern side of the roof?" and therefore avoid most of the shade altogether.

 

That's the first, and most important rule about shade. Avoid it where you can.

The picture below is taken at 3.30pm in the middle of Summer and we can see the shade on two of the panels. What a shame with all that space at the other end of the roof.

 

 

At 3.30pm in Summer if there were no shade on the panels at all, and assuming it were a typical 6.6kW install with two strings of 11 panels, then the output of the solar would be expected to be 1.25kW from each string, so 2.5kW in total

 

With two panels shaded on one of the inverter strings this would likely bring the output down to 0.6kW on one string and 1.25kW on the other, so in total, 1.85kW of power.

 

They have lost 0.65kW of power. Maybe a full 1.5kWh of lost production by the end of the day. Wait a moment, I hear you cry, "So what? That's worth almost nothing". Exactly right. It isn't worth much, and that's why spending huge sums of money on technology to beat shade is often wasted if the power loss happens before 9am or after 3pm.

 

However, as this FAQ is also about available technology, let's look at that.

 

If we assume that this homeowner has plans for the other end of the roof so the solar couldn't go there, then one of the things they could have done is fit some DC optimisers on the panels that get some shade.

 

DC Optimisers adjust the voltage of the solar panel they are connected to up and down as shade hits the panel, and by doing so the remainder of the string of panels are unaffected. At $90 a pop for a few panels they are a very cost effective solution. Huawei have their own, but the Tigo brand of optimisers can be 'selectively' deployed too on any inverter (e.g. Fronius, SMA, Goodwe, Growatt, Sungrow etc). If you want to get information about how effective these optimisers are working, you can buy an extra component that reports on the individual output of every optimised panel.

 

SolarEdge have an inverter that can't even operate unless every single panel has an optimiser fitted. This is pretty extreme, and expensive, but its a popular brand so many people must like the idea of seeing the ouput of every panel in their reports.

 

Micro inverters, and in Australia there is only one available brand, Enphase do the same job as DC optimisers as far as shade is concerned, and they also have some extra features that appeal to some homeowners. Like SolarEdge they need to fit a micro inverter on every single panel, and by doing so, every panel is a law unto itself. It converts the panel DC voltage into AC voltage there on the roof so all of the panels are producing 240V AC and each additional micro is adding about 1 Amp of current.

 

Diodes are built into every solar panel and until recently these three diodes controlled typically 3 strings of 20 solar cells each inside the solar panel. If the panel was partially shaded the diode would bypass those cells allowing the rest of the panel to carry on without losses. The latest half-cell panels take that a step further with 6 strings of 20 half-cells further improving the panels ability to minimise losses from shade. That is free tech, built into your solar panel.

 

Typical power losses from shade

In the case of the system above, if the house was entirely unshaded the whole of the year, with 6.6kW of panels facing North North East, as it does, the output of the system would be 32kWh per day on average after the first year (11,670 kWh for the year) and then a very small reduction on that each year thereafter (0.3 to 0.6% depending on the cell type, 'P' or 'N' but that's another story).

 

Having studied nearly ten years of images of this particular home, and comparing the outputs of other installations that we have done (we didn't do this one), I would be confident that the annual output of this pictured sometimes shaded solar installation would be about 20% less than that, or 26kWh per day (9,490kWh) in the first year.

 

Their shade will lose them 6kWh per day.

 

If they had optimisers or micro inverters on all of the panels then the output would likely go back up to 30kWh. No amount of tech is going to get them all the way up to 32kWh because no matter what protection is given to the unshaded panels, the output from a shaded panel is going to be down.

 

If the additional cost of adding micros or optimisers is $2,000 and it recovers 4kWh per day for them, then what is the value of that 4kWh? If the extra 4kWh was exported as surplus then $104 a year, consumed it then $420. As it is impossible to know what this particular house consumption is between 8am and 5pm (solar production), its impossible to say whether it would be a waste of money or not. We can be a lot more specific with YOU because you can easily work out what your daylight hours power use is by a bit of simple meter reading.

 

Pictured below, Huawei DC optimiser.

 

 

 

 

 

 

 

 

 

 

 


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