Solar System Shading: What to Know

Despite the abundant number of variables involved in estimating the production of a solar system, the overall process is conceptually quite simple: 1) estimate the amount of sunlight (photons) reaching a solar system and 2) compute how much of that sunlight will be converted into usable electricity (electrons). Shading can have a significant effect on both of these calculations. This article will discuss non-problematic shade sources, site-specific shading obstructions and their effects, technologies that mitigate the effects of shading, go through a couple case studies, and give recommendations for homeowners whose systems may experience shading. Many sources of shade that system owners worry about can be easily accounted for in scientific, accurate ways and should not be a cause of concern. These include things like cloud cover, fog, haze, system orientation, and system slope. A solar system that faces east on a 45° slope will get more sun in the morning than it will in the afternoon. Similarly, a system in Seattle is going to lose more sunlight due to cloud coverage than one in Hawaii. Because the path of the sun and weather patterns for particular areas are known variables, they will typically be the starting point from which other factors are applied. This baseline is what is referred to as “available sunlight”. Sun map Figure 1: An example of an irradiation map showing sunlight potential on different parts of the roof Shading obstructions are site-specific sources of shading that reduce a solar system’s available sunlight. These can include things like trees, chimneys, other parts of the roof, mountains, and nearby buildings. A system’s available sunlight will be reduced by an amount approximately equivalent to how much it is shaded. As an example, a system where 50% of the panels are shaded for 20% of the day will have an overall reduction in potential sunlight of around 10%. A notable exception to this rule is that shading in the early morning and late afternoon tends not to matter much as sunlight is not very intense at these times anyhow. It bears mentioning that estimating shading in the real world is much more difficult than the contrived example above. The sun follows a different path across the sky every day of the year and sits at a different point in that path every minute of the day. Shading obstructions that are problematic during one part of the day or time of the year may be trivial during another. Sun graph  Figure 2: Site-specific shading sources block sunlight that would otherwise be available. Source: ‎Lumos: Sun and Moon Tracker In addition to the effect of shading on available sunlight, which is fairly intuitive, shading can also have a significant effect on how much sunlight that reaches a system actually gets converted into electricity. Gil Masters of Stanford University does a nice job demonstrating this effect in his book Renewable Energy and Efficient Electric Power Systems by showing how shading one cell out of 36 can reduce power output by more than 75%. So, while shading 10% of a system’s available sunlight will reduce the overall production by no less than 10%, it can actually eliminate nearly all of the system’s production. Explaining why shading can have such a disproportionate effect on the output of a solar system first involves understanding how electricity works in general. One of the most useful analogies for doing this is to think of an electrical system as a waterflow system. Just as a kink in one part of a water hose will affect the amount of water flow in every other part, so will shading in one solar cell or solar panel affect the electrical flow in every other part of the system. Flow disruption infographic                        Figure 3: Individual component issues can have outsized effects in both electrical and water flow systems The second piece of necessary understanding is how a traditional solar system works. Solar panels are comprised of solar cells, typically either 60, 72, or 96 of them. The cells are the actual component that converts sunlight into electricity. These cells are strung together in one long series, which is the reason one bad cell can bring down the whole group. Traditionally, panels are then wired together in series as well, usually 1-4 groups of them that go to a single inverter, which is the device that converts the DC electricity produced by a solar system into the AC electricity used by a building. The flow of electricity is thus going through every panel in a string and every cell in all of those panels. This introduces lots of opportunities for kinks in the system. Also, because they will typically be going to one inverter, one bad cell/panel will not just bring down the production of the string it is in, but in the inverter’s other strings as well. This is one of the main reasons why traditional string inverter systems are becoming less common. The primary technologies that exists to mitigate the effects of shading within solar panels include bypass diodes and half-cut cells. A typical solar panel has three bypass diodes, which enable a poor performing group of cells to be “skipped over”. In this case, a shaded cell can only bring down one third of the panel, rather than the whole thing. Half-cut cells take this to another level by separating the panel into two parts, halving the effect of shading from one third to one sixth. Solar cells          Figure 4: Half-cut cells and bypass diodes can help mitigate shading effects for individual solar panels The main technologies for mitigating shading outside of solar panels are MPPT and MLPE. Strings of solar panels can operate at a large range of voltage. Depending on the amount of available sunlight, there is a particular voltage that will generate the greatest amount of power. Power Point Tracking (PPT) refers to an inverter’s ability to set a string or a group of strings’ voltage at this sweet spot. Some inverters can independently set voltages for two strings or groups of strings, which is referred to as Multiple Power Point Tracking (MPPT). This can prevent one shaded string on an inverter from hampering output on all the other strings. However, because a few bad cells can still bring down half of a system’s output, MPPT’s ability to combat shading is still very limited. A much more robust solution to mitigate system shading are Module-Level Power Electronics (MLPE). MLPE comes in two forms: microinverters and power optimizers with Enphase and SolarEdge being the leading manufacturer in each respective category. Microinverter systems use a large number of rooftop inverters, typically one per solar panel, to convert the electricity from DC to AC. Power optimizer systems use rooftop units to condition the DC output of individual solar panels for conversion by the string inverter. Inverters graphic                                                     Figure 5: MLPE technology can help mitigate shading effects for groups of solar panels Both MLPE systems maximize the output of individual solar panels and prevent underperforming panels from dragging down the output from the rest of the system. They also both provide more granular level production data by facilitating panel-level monitoring. Microinverter systems have the added benefit of preventing a failed MLPE or inverter from disabling production for the entire string or system. Solar panels on roof                                                                Figure 6: Case Study #1: Metal chimneys are a common source of shading Case Study #1 Case study #1 provides an example of intraday shading. Notice how the obstruction, which has significantly less volume than the panel it is shading, is reducing output of the panel to its north by >90% on January 21st. Later that same day, the shading has moved to the panel at the obstruction’s east, reducing the production of it by just under 90%. These scenarios show the powerful mitigation effect of MLPE as the production of this system using a string inverter would be significantly reduced. Power usage chart                                   Figure 7: Case Study #1: 12:00N: Microinverters prevent this shaded panel from bringing down the whole system Solar usage chart                                         Figure 8: Case Study #1: 3:50PM: The shadow has moved to the chimney’s east as the sun begins to set Case Study #2 The second case study provides an example of a system that is relatively close to a tree line. Notice how the panels on the bottom row (the ones that are closest to the tree line and farthest down in elevation) are only producing 63% of those on the top row during the month of January when the sun sits low in the sky. The effect is much less pronounced (94%) in the month of August when the sun sits higher in the sky. Solar usage chart                                            Figure 9: Case Study #2: Tree lines to the south of solar systems cause lots of shading in the winter Solar usage chart Figure 10: Case Study #2: Trees lines to the south have less effect on summer solar production There are three takeaways for homeowners who are worried about shading on their solar systems. Takeaway #1 is to be realistic about how much sunlight the area is getting. If a rooftop only gets full sun in the summertime between 11:00am-2:00pm, there is simply not enough sunlight to make solar a viable option. Prospective customers will sometimes see solar systems covered in shade and assume that their home is a good fit. Systems are sometimes installed in the shade at the insistence of customers. Most of the time, unfortunately, heavily shaded systems are the result of unscrupulous installers who ignore or downplay the dramatic effects shading can have. Takeaway #2 is that systems with site-specific shading obstructions should use MLPE inverter technology. This applies to pretty much every system that gets any amount of shading from 8:00am-4:00pm. The initial cost savings of not using MLPE will be far outweighed by the production losses over the life of the system. Takeaway #3 is to not differentiate solar installers based on their production estimates. Solar production is a critical number for estimating system payback, return on investment, lifetime savings and all the other great financial benefits from going solar. However, using production figures to choose an installer encourages installers to exaggerate production figures and leaves homeowners disappointed in the system and provider they chose. For advice on how you should compare solar quotes, please see our blog, The Expert Guide to Comparing Solar Quotes. 3 Takeaways

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