Why Solar Panel Shading is a Big Deal
Daniel Mark Schwartz
October 18, 2019
Shade is the enemy of solar panels, but common in lush green homesteads and farmsteads of the off grid community. Even small amounts can destroy power output of solar systems. Let me walk you through what shade does to your solar system.
Do solar panels work in the shade? While solar panels do work in the shade, even partial shading of a panel can significantly decrease power output compared to full sun. My experiments have shown that just small patches of shade can reduce solar power output by 90% in some panels.
How does shade affect solar panels?
In general, the less light the falls on a panel, the less power it produces. Panels are designed with ratings for what they would produce on sunny days with clear skies. However, in cloudy or shady conditions, solar panels will produce much less energy.
According to the graph shown below, this decrease in the maximum available energy produced by a panel is approximately linear compared to the intensity of light hitting the panel. For instance, a panel rated at 100W in the sun will only produce 50W if the intensity of the light hitting it decreases by half.
Shade and overcast days usually have a dramatic effect on the brightness of the light hitting solar panels, and thus their total power output. According to a study produced by researchers at the National Bureau of Standards and UCLA diffuse (indirect) light measured on sunny days can frequently be as low as 10%, which means that panels shaded from direct Sun may only produce a tiny fraction of their total output power. Thus, your 100W panel would be expected to only produce around 10W.
What is partial shading?
Another type of shading that can effect your solar array is partial shading. Partial shading is when a panel or array is struck by light of varying brightness, such as half the paneling being in shadow or stippled light filtered by tree leaves.
Unlike constant shading due to complete shade or cloudy /overcast days, partial shade is much worse for solar panels than you might expect. In experiments I have personally seen the shade from my hand completely cripple the output of a 100W solar panel to just 10W. Even when the rest of the panel is in full sun.
This seems strange at first glance, but after a little more in depth look at solar panel internal construction makes it clear why this is the case, and what we can do to improve the situation.
On the surface of a solar panel you will notice a grid of dark squares, sometimes with a little space between them. These are the individual solar cells, which produce the rated current (amps) but only about 0.5V each. In order to increase the panel voltage up to the rated potential, these cells are connected in series, like Christmas lights. For a typical “12 volt” panel you will have 36 individual cells connected to produced about 18V open circuit voltage.
Although necessary for the panel to operate, the downside to connecting all these cells in series is that low power output in one cell effectively blocks power from all the other cells in the string. That’s why, if a hand covers up only one of the squares, the whole panel can produce only a fraction of it’s output potential.
What are bypass diodes?
In order to help mitigate power loss due to partial shading, most solar panels include internal blocking diodes. Diodes are essentially electrical one way valves. Under normal operation, bypass diodes do not harm or help the panel’s power output. But, when some of the solar cells are shaded, reducing voltage of the internal string, the diode allows current to bypass the affected string and allow the panel to continue producing power.
You will see the diodes signified in the above diagram by the triangle and line symbols near the top of the panels. Most solar panels include three internal bypass diodes, effectively dividing the panel in to three internal strings — left, right, and middle.
When one of these strings is shaded or otherwise incapacitated, the diode allows the current to pass it by, cutting the cells out of the system and diminishing the total output potential temporarily by 1/3rd.
Additionally, if you have several panels connected in series, a practice which I generally recommend for off grid solar systems due to cost considerations, partial or complete shading of one panel will also diminish power output of other panels in the same strings as well. These three internal diodes also serve to cut out entire panels if they would reduce power generation, allowing the other panels to still function.
How much could shading effect Solar Efficiency?
The effect of solar shading depends a lot on the configuration of your panels and the amount of shading. Full shading can reduce panel output by 50% – 90% compared to direct unclouded illumination. Cloudy days can commonly reduce solar panel output by as much as 90%, leaving 10% total rated power output of the panels.
If you have the option to move your panels to an un-shaded position, it is often worth the money to make the move. Panel shading is a serious concern and often results in needing to buy significantly more panels, invest in more efficient electric appliances, or reduce power consumption.
Do solar panels work in partial shade?
While solar panels do work in partial shade, their output is significantly reduced. In some cases as much as 90% by even small amounts of shade. Although, correct panel configuration, and usage of bypass diodes can help mitigate power loss, the best option is usually to move your panels out of the shade.
When operating solar panels in heavily shaded conditions, you can potentially get a lot more power by including per panel MPPT controllers, which can work to get the maximum power out of your off grid panels, when other simpler connection methods would just cut out the panel.
Do solar panels work when cloudy?
Yes. Solar panels work while under cloud cover, but have much reduced power production. The amount of power your panels can produce depend a lot on the climate in your area, typical panels produce 80% – 20% of their rated power.
Depending on the design of your panels, solar panels in cloudy conditions may work extra inefficiently if used with PWM charge controllers. Users report that smart MPPT charge controllers can wring more power out of dimly lit panels under cloudy conditions.
Which is better monocrystalline or polycrystalline panels for shaded and cloudy conditions?
There is not a lot of information on this subject on the internet. According to marketing material, monocrystalline panels are more efficient at converting light to electricity, but because they also are smaller for a given power rating, in the end monocrystalline and polycrystalline panels should produce the same amount of power given the same light conditions.
However, because increased size, and thus increased cooling, poly solar panels can often perform better in full light conditions than their more expensive mono counter parts. For this reason, I generally recommend off grid solar arrays use poly panels, unless they have sever space limitations such as an off grid RV or portable tiny house.
Do solar panels work when raining?
Solar panels do work when it is raining, but because dark, over cast skies that usually come with rain, the power output will be much reduced compared to a relatively bright and sunny day. Power output as low as 10% of rated max are not uncommon on dark rainy days.
What About Module Level Power Electronics (MLPE)
Module level power electronics are a relatively new idea to help increase the power output of solar arrays. Essentially, they are a tiny charge controller (usually MPPT) and possibly inverter that is connected to and only operates one panel of your array at a time. That means, every panel of your array would have it’s own smart controller constantly adjusting it’s power output for maximum efficiency.
In conditions where only some of your solar panel are shaded, especially when the shade is constantly changing, MLPEs can be much more efficient than only connecting one or two system level controllers. Especially under partial shade conditions, each panel may have it’s own maximum power point, and by assigning a controller to each panel, you may increase the total array output over a system wide controller, which can only find the average best power point of the whole array at once.
Each MLPE includes it’s own DC-DC and/or DC-AC converter which means that you could choose the panel output potential essentially independently of the panel’s natural voltage. Thus, there would be no need to wire panels in series, further reducing potential power loss to partial shading, while still allowing reduced wire costs due to higher voltage panel array configuration.
For example, you could run your “12V” panel through a MLPE that outputs 72V, wire all the panels in parallel for maximum efficiency, and then down convert your power using dumber DC-DC convert / charge controller (no need for MPPT capability) at the battery bank if necessary.
Of course, MLPEs are a relatively new commercial concept, and their biggest downside is the overall cost. MLPE electronics are not cheap, and it remains to be seen if their increased efficiency would out weigh their high cost. Probably, it is still cheaper just to buy a few extra panels rather than outfit your entire array with individual charge controllers. But, in a few years things might be different.
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