Solar PV Panels and Shade

There’s no doubt about it: Shade is not good for solar PV panels. However, there are a lot of misconceptions about the true effects of shade. So let’s clear up the misconceptions. Knowing more about solar and shade will help you design a solar PV system that’s both more productive and more cost effective.

Christmas Tree Lights

Do you remember the old-fashioned Christmas tree lights that would all go out if one bulb failed? A common myth about solar is that if any one of your solar PV panels gets shaded then they will all stop working, like the Christmas tree lights. Thankfully, this is totally false. What actually happens is that the shaded panel will be bypassed while all of the other panels continue to work as normal. This works because of something called bypass diodes:

Solar Panels and Shade | Bypass Diodes

The Three Bypass Diodes

Most solar panels today have three groups of cells with a bypass diode for each. These bypass diodes let electricity pass by the shaded cells so the others can continue to work properly. And there’s the moral of the story: Shade on one of your panels will not stop all of the others from working.

A sting of solar PV panels connected to a string inverter.  The image shows the use of bypass diodes to ensure that shading of one panel or group of cells does not prevent the others from working.
The three bypass diodes make sure that even if one group of solar cells/panels is shaded, the rest can continue to operate as usual. Image license CC-BY. Credit: energyd.ie

Partial Shading

Here’s where things get a little more complicated. Let’s say one of your panels gets light shade, so that it can only produce about 50% as much power as the other panels. One one hand, the bypass diodes can kick in to make sure that this doesn’t affect the other panels in the string. However, you probably don’t want to lose the 50% of power that the panel still can produce. This is where two other options might come in useful: Power optimisers and microinverters.

Power Optimisers

A power optimizer is a small device that is fits behind a solar PV panel. The purpose of the power optimisers is to make sure that each PV panel operates at the optimal power output regardless of what the other panels are doing. Sounds great, doesn’t it? In reality, your PV system won’t gain very much power by using solar PV optimisers. An independent study found that optimisers give a boost of around 4.6% in systems with lots of shade. Unless you’re limited by space you’ll generally be best off spending your money on an extra panel instead. Also, make sure choose a reputable brand of optimiser. Not only will replacing a faulty optimiser be costly, but it can also be a difficult job due to the location of the optimsiers.

A sting of solar PV panels with power optimisers.
Adding a power optimiser to each panel means that all panels can operate at their own optimal power output, regardless of shade and other panels in the string. Image license CC-BY. Credit: energyd.ie

Microinverters

Most solar PV systems use one inverter for a whole string of solar PV panels. On the other hand, with microinverters, you have multiple smaller inverters, each connecting to only one or two panels. Just like with solar PV optimisers, this means that each panel can continue to operate at its own maximum power output, regardless of what the other panels are doing. Microinverters are well worth considering, especially because they come with a number of additional features and benefits. As for power optimisers, be especially careful to choose a reputable brand if you decide to use microinverters.

Separate solar PV panels, each with their own microinverter.
With microinverters, each panel operates independently of the others. Image license CC-BY. Credit: energyd.ie

Conclusion

Shade reduces the power output of your solar PV system. However, the idea that shade on one panel will stop all of the others from working is a myth. Still, you can use optimisers and microinverters to minimise power losses associated with partial shading. On the other hand, it’s often more cost-effective to add one or two extra solar PV panels than to use power optimisers and microinverters.

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