Solar and Shade - Microinverters vs String Inverters
Solar
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September 6, 2021

Will my Fronius inverter still work in the shade?

There are many schools of thought and divisive opinions about the ‘right’ and ‘wrong’ solution when it comes to 'solar and shade'. Retailers, installers, suppliers, marketing, and folklore amongst the industry all play their part, and with so many differing opinions, it’s easy to understand why these mixed messages cause so much confusion.

There has been growing discussion around the benefits of installing panel-level optimisation over string inverters, in both shaded and non-shaded environments. Eddy May, NRG Solar Managing Director, set out to determine how solar production varies when using different inverters with shade as a major factor.  His three-month testing exercise, which involved erecting a purpose-built, ground-mounted testing station, also illustrates how far solar panels and inverters have come in the past decade.

 

Testing Rig in the NRG Solar Carpark

What is panel-level optimisation?

It is often believed that in a conventional solar power system, if one panel is underperforming, all other modules will be dragged down too as they are connected together in ‘strings’, hence the term ‘string inverters’. The result of this is, if one solar panel is shaded and operating at 50% all panels in the same string will also only operate at 50% - or at least that’s what we’re told.

Panel level optimisation (or PLO) uses smart electronics, like microinverters and optimisers, to ensure each panel can pass on the maximum current without any interruption - but how necessary are these added extras?

Putting it to the test - microinverters vs. string inverters

In the middle of the NRG Solar car park, a test rig was erected to allow the team to test several different scenarios that often impact the performance of a ‘string’ solar panel system. Testing began in May to cover Adelaide’s temperamental Autumn and Winter conditions with results collected at the beginning of August.

The test rig had two identical solar arrays - one with Enphase microinverters and one with a Fronius inverter, testing the performance of each system in various conditions. The setup was on one single-phase power supply to avoid voltage imbalances.


The different tests Eddy conducted included:

- solar production with no shade, swapping the arrays with inverters to prove equal conditions

- simulation of a 'roof vent pipe' shading each array

- simulation of a ‘chimney’ shading each array and fact-checking various marketing materials still currently being used

- smashing one panel on each array with a cricket bat to test the impact of micro-cracks.

 

What can we learn from this?

As Eddy explains it, the consensus in the solar industry suggests that if you have chimneys or any shading, you would generally have to opt for microinverters or optimisers - otherwise your solar system will not be adequately efficient. Microinverters and optimisers are said to allow panels to perform on panel level, meaning that if one panel is shaded, it will not impact the rest of the array.

Similarly, the school of thought is that string inverters would be unsuitable in this situation and, due to the string-make-up, it would mean the entire array would be adversely impacted by the one shaded panel.

It is widely thought that when designing a solar system, you should absolutely steer clear of using a string inverter if there is any possibility of shade coverage, no matter if it’s as little as a chimney over a single panel. Yet, the evidence suggests otherwise.


So, what was the outcome of the chimney test?

When Eddy tested the arrays with a 'chimney' simulation, the Enphase microinverters outperformed the Fronius string array by about 2% - 4%.

But, during the sunniest day, Fronius outperformed Enphase for most parts of the day. Which, as Eddy explains, goes against everything the solar industry thought about string inverters and shade.

 

So what are the key findings?

Eddy summarises his testing and findings in this video: 

Enphase’s panel level monitoring is exceptionally good. Whilst we used Solar Analytics as the unbiased ‘umpire’ to determine all the outcomes, the Enphase panel-level monitoring highlighted what was going on behind the scenes on the Enphase system.

In a nutshell, Enphase microinverters performed marginally better in some situations, but there were also times when Fronius performed better.

The results with shading, chimneys, or micro-cracks are about the same. If it is mostly clear, which is about 60% of the year in South Australia, the overall difference between the microinverter and the string inverter is about 1%-2% . If it is overcast or rainy, this escalates slightly to about 3-4%.

 

Eddy taking a cricket bat to the panels

Some other tests conducted which will allow us to give better advice to industry system designers included;

o   Oversizing the string inverter to make up for the lower productions from the Fronius

o   We saw that adding one additional panel to the Fronius array made that array perform better than the Enphase system.

o   Having two strings in parallel (one unshaded and one heavily shaded and included the broken panel) into the one mppt on the Fronius. The maximum we could get the Fronius to deteriorate in this condition was 19% worse than the microinverter system. This proves that care still needs to be given to the design.

 

The Result

“As expected, Enphase ultimately came out on top in most situations. But when we look at the data, the Fronius string inverter was only a few percent behind, clearly proving that high-quality solar panels and string inverters like a Fronius have in fact come a long way to mitigate the age-old problem of shade."

Disclaimer – Eddy is not saying to go and build a solar system in the shade – direct sunlight is needed to create the best performance from your solar system. Eddy and the NRG Solar team continue to design and build solar systems using Enphase and Fronius.

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