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Why even partial shading is bad for solar power systems

by James Martin II on May 10, 2012

in Installation advice,Positioning solar PV panels

Although it probably goes without saying, shading is not good for solar panels. A shadow cast on even just part of one solar panel in your solar array can potentially compromise the output of the whole system. What are some strategies for dealing with potential shading of solar arrays?

Why are solar panels averse to shade?

In most instances, solar photovoltaic (PV) systems for homes and businesses consist of solar panels (the collection of which is referred to as the ‘array’) and an inverter. The solar panels catch sunlight and convert it into DC (direct current) electricity, and the inverter in turn converts the DC electricity into grid- and appliance-compatible AC (alternating current) electricity.

Most small-scale solar systems, such as a 1.5 kilowatt (kW) system, for homes and small businesses will include at least 6 panels. However, systems can easily be composed of more panels where necessary, depending on the output of the panels in question and the electricity demands of the occupants of the building they supply power to.

For technical reasons related to the voltage requirements of the system’s inverter, solar arrays are usually divided into ‘strings’ of solar panels. Small systems may only have 1 string, while large systems could have many more. One string could consist of a single panel, but usually they have more.

You can think of a string of panels as something like a piece of pipe, and the solar power is like water flowing through that pipe. In conventional solar panel strings, shade is something that blocks that flow. If, for example, shade from a tree or a chimney is cast on even one of the panels in the string, the output of the entire string will be reduced to virtually zero for as long as the shadow sits there. If there is a separate, unshaded string, however, this string will continue to produce power as per usual.

In extreme cases, a shadow does not necessarily need to fall on an entire panel–depending on the technology used in the solar panel in question, shading of even just one cell could flatten the output of the panel and in turn the entire string. Many modern panels, however, come equipped with devices called bypass diodes which minimise the effects of partial shading by essentially enabling electricity to ‘flow around’ the shaded cell or cells.

Strategies and technologies for dealing with shaded solar panels

Although the performance and therefore the return on investment (ROI) from a solar power system can be severely affected by shading–especially shading that occurs regularly due to an object that casts a shadow at the same time every day as the sun passes through the sky–there are a number of ways to avoid or mitigate these effects.

Site your solar panel array where there will be no regular shading

This is the first and most obvious step to making sure your system does not suffer the consequences of being partially shaded. It is extremely important to consider all times of day for all seasons of the year when working out whether some nearby object might cast a shadow onto your roof. You can check this yourself or alternatively your Solar Choice broker will check to ensure there is no shading on your roof using a program called Nearmap.

shaded solar array

An example of a tree which has grown tall enough to cast a significant amount of shade on the solar array of a nearby roof.

Solar system owners should also be vigilant in making sure that there are no nearby trees which might grow tall enough to eventually cause shading issues. Solar system lifespans are typically expected to be 25+ years, during which time trees have plenty of time to grow.

Clouds are another source of potential shading. Clouds passing through the sky during the day may also result in fluctuations in system output, but these are basically unavoidable. Amorphous silicon solar cells are said to be better at handling shading than crystalline silicon solar panels, but generally speaking the relatively low overall efficiency of amorphous panels means that crystalline modules are a better choice.

There are some other technologies under development that may offer high efficiencies even in inclement weather, such as ‘super black’ solar cells, but most of these are still either expensive or not yet commercially available.

Use an inverter that has MPP Tracking capability

Maximum Power Point Tracking (MPP Tracking or MPPT) is a technology that now comes standard in most quality inverters. An inverter equipped with an MPP Tracker is able to ‘average out’ the difference between the current and voltage of 2 or more strings of panels, so that even if the output from one is sub-optimal, the system is still able to take advantage of whatever juice it is creating and add it to the more powerful string, producing a consistent, usable volume of power. Inverters without MPPT capability simply lose the output from the weaker string once it passes below the required output threshold.

MPPT and shading in a multi-string solar power system

An example of partial shading in a conventional multi-string inverter system (Image credit: pvsolarchina.com)

There are also a number of companies (such as SolarEdge) who offer module-level MPP Tracking technology. These inverters offer MPPT for each individual solar panel, averaging the output of all panels more ‘intelligently’ than a central inverter, which can make adjustments only at the string level.

Enecsys is another company that produces a type of mirco-inverter. Enecsys micro-inverters sit on individual modules and converter all power from each panel directly into AC electricity, avoiding power losses from shaded strings. Tindo Solar panels are an Australian-made solar panel brand that come equipped with Enecsys inverters.

© 2012 Solar Choice Pty Ltd

James Martin II

James has been working as analyst and online development manager for Solar Choice since 2011 and has contributed hundreds of articles to the Solar Choice website during this time.

He holds a master's degree in Environmental Management from UNSW, and a bachelor's degree in Philosophy from Bridgewater State University in his native Massachusetts.

He currently works remotely for Solar Choice from New York City.

{ 14 comments… read them below or add one }

Ron June 18, 2012 at 10:53 am

Hi, What an excellent website. 2 questions please.
Q1 My shading problem appears minor. I have 4 rows of north facing panels on angle brackets all at 10 degrees. As we approach the winter solstice I notice about 100mm of shading falling on row four from the elevated edge of row three. It is a result of the morning slant sun angle and by 10am the shadow has gone. Would shadow shut off the panel or array whilst shaded? I have a Sunny Boy SB 3000TL-20
Q2 If I increase angle of row 4 to 20 degress will the aimed improvement be held back by other rows which remain at 10 degrees?
PS I don’t follow your solar-tilt drawing as it seems to suggest that increasing tilt angle from 10 degrees will decrease performance. Probably I am not reading it correctly? Help!

Reply

admin July 9, 2012 at 4:44 pm

Hi Ron,

Thanks for your questions.

Q1. This narrow band of shading could be affecting the output of your array depending on a) how your panels are arranged (i.e. how many strings) and b) the specifications of panels you have.

a) If you have multiple strings, the shading may only affect one of them–i.e. the panels in row 4, if they constitute a single string. If rows 3 and 4 are on the same string, however, the shading could potentially be cutting into the output of all of them, as in most conventional panels the ‘weakest link’ breaks the chain.

b) Some panels have bypass diodes that handle the problem in solar panels where even partial shading has a disproportionate effect on overall array output. Such panels will only see a loss equivalent to the number of cells that are shaded (or possibly slightly more depending on the number of bypass diodes) instead of knocking out the entire string.

A couple more comments: Do you have a means of monitoring your system output? If you do, you should be able to see what effect this partial shading is having on your system’s output. Additionally, consider yourself lucky that the shading is coming from a lower row of panels instead of from a tree on a neighbour’s property, which would keep growing!

Q2: Changing the angle of row 4 will again depend on how many strings your array is divided into. If it is in its own string and plugged into the MPP tracker on your inverter just like that, it shouldn’t be a problem to change the angle. I don’t know where you’re located, but I would assume that you’re in a tropical area given the low angle of your array? Was it a flat roof originally? If you’re anywhere outside a tropical area (and still in the southern hemisphere), it’s generally better to have the panels tilted at around 20-30 degrees from flat, facing north. If you do increase the angle of the last row of panels and it happens to be on the same string as row 3 and you latitude below about 30 degrees south, this should actually improve the output of that string–albeit only slightly. A 20 degree angle will catch more sun further south.

Hope this make sense. Good luck!

Reply

william September 7, 2012 at 9:48 am

we have a 1.5k solar system facing west vitally unshaded until late afternoon.in 18 months we are credited by supplier with about one(1)kw per month.how do we know that the feed in system is working?the installer seems to think we have the best system due to the higher tariff credit.if it isn’t working correctly it is a poor return on cost.is there a display in the metering system we can access?we would be more comfortable if we were able to assess any savings other than the lousy 1k.per month.

Reply

admin September 10, 2012 at 11:13 am

Hi William,

the fact that you’re feeding out 1 kilowatt-hour (kWh) per month could mean either that your system isn’t producing, or that you’re using all the power that your system is producing as it is being produced. If it’s the latter, that’s not necessarily a problem, as it means that your system would still be saving you money on your electricity bill.

If there is somewhere to see what your system is producing, it would likely be on your inverter. Some inverters even have wireless access so that you can see how your system is performing online. Do you have a user’s manual for the inverter?

If you believe that there is indeed a problem with your system, you’d be best advised to contact your installer to come out and have a look.

Best of luck!

Reply

tony jacques September 27, 2012 at 4:15 pm

My question re shading comes from the opposite side. I currently have 12 x 170W panels and a 2kw inverter. Inefficiencies result in generation way below inverter capacity – especially in winter. Physically I can add a further 6 x170W panels which will boost me in winter (but not exceed the inverter capacity). However in summer with, say, 80% efficiency my 18 panels will produce about 400W OVER inverter capacity. Can I bring the output down below inverter capacity by deliberately covering,say, two panels during extreme summer generating periods ? I envisage eg a sheet of ply, sarking or the like.

Reply

Solar Choice September 28, 2012 at 5:31 pm

Hi Tony,

I’ve spoken to our in-house Solar Engineers and they have advised me that adding extra panels would be very detrimental to your current set up. Although most inverters are set up to cope with energy production over the official capacity this essentially a safety measure and should not be used on an ongoing basis.

They also strongly advise against covering your panels. Shaded or covered panels will heat up, this in addition to the sun hitting your roof heating up and drying out what ever material you have used to cover the panels could result in a fire.

If there is some way you can remove the source of the winter shade this would be a less costly and safer alternative.

We hope this answers your question

Reply

Tim Hughes December 13, 2012 at 4:37 am

Hi. I have a 2.2kW system made up of 9 panels in 1 row and in one array. However the 9th panel is in the shade to such an extent, and stopping the other 8 from working, that I am thinking I might actually be better without it. Is there anything else I could do?

Reply

Solar Choice December 13, 2012 at 9:36 am

Hi Tim,

As solar panels are connected through a ‘string’ shading on one panel will negatively impact all the others as you’ve seen. I would check your installer contract as some include a clause about moving panels for free if they are not generating the optimum amount of energy, but this would probably be if the panel has always been shaded. If the original installer placed the panels in an obviously shaded area it may be worth while paying an other installer to relocated them.

If the panel is shaded as the result of tree growth our suggestion would be to prune the tree allowing the light to get to your panels again. Alternatively you could have them relocated to a shade free part of your roof, this has to be done by a qualified installer or it may void the warranty.

A good installer should have accessed for any shading, we are aware of a few installers (who are not on our network) who actually have images on their websites of completed jobs wshowing the system in full shade or, in one case, shade created by placing mounted panels too close together! If you feel you may have an issue with the original installer you could talk to another company. Look out for evidence of a good company history (they will normally be happy to talk to you about this to reassure you), Clean Energy Council accredation and they may be able direct you other jobs they’ve done in the local area so you can speak to exisiting customers.

Hope this helps!

Reply

Don August 6, 2014 at 12:54 pm

Hi James
In your section titled “Use an inverter that has MPP Tracking capability” you said in line 4 “the difference between the current and voltage of 2 or more strings of inverters”. Should this say solar panels, not inverters?

Reply

Solar Choice Staff August 7, 2014 at 4:39 am

Absolutely, Don. Thanks for pointing out the typo–fixed now.

Reply

Sam August 14, 2014 at 12:17 am

I have a 44 panel 11 kW single file PV array, and a neighbour is applying for planning permission for a wind turbine which will throw a shadow on to my array at certain times of the day. The turbine will be sited so the entire length of the array will be affected. Will this ruin my output completely? What can I say to the Planning to let them know this is very bad for my system?

Reply

Solar Choice Staff August 14, 2014 at 3:48 am

Hi Sam,

Unless you’ve got dc-dc optimisers or microinverters the shading will definitely affect the whole array, which I’m assuming is a single string. Even if you have those, it sounds like the neighbour’s turbine could affect your system’s output.

You could check to see if your planning board/council area has any rules about not blocking a neighbouring property’s solar access (unlikely). Otherwise you could try to base your case on the damages that you would suffer if the turbine were to be built–I’m sure you could easily quantify what your losses would be.

In any case, hopefully you’ll be able to negotiate with your neighbors so that they can site the turbine in a different location that doesn’t block the sun for you.

Best of luck with your system.

Best,
James

Reply

Adel September 14, 2014 at 10:37 am

Hello James; thanks a lot for this very interesting post.

I have a 2Kwp system, 20 x 100 kwp 12V panels; each 4 are connected in series, hence having 5 strings.
These 20 panels are installed on 2 lines, 10 units each, and they are equipped with diodes.
My system runs on a Steca Tarom 4055 MPPT charge controller, and it was installed 3 days ago. It’s an off-grid installation with 48V batteries.

After reading the above, I would assume that this entire system goes down because of a horizontal shade that blocks cells in multiple modules.
However I have noticed during the 2 tests I have made so far that the charger is displaying on its screen an overcharge message, with a voltage of 56V instead of 54 (the max I have set in Tarom).
And the output isn’t working.
I have to open the junction box and open / close all strings’ fuses to have it adjust and work again.

Did you encounter any similar situation?
Do you think this overcharge is due to the shade on panels?

Best,

Reply

Solar Choice Staff September 16, 2014 at 6:13 am

Hi Adel,

If I were you I would get in touch with the installer who designed and set up the system–the overcharge warning doesn’t sound right, especially if you’ve just had the system installed a matter of days ago!

Reply

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