A solar system may appear to be well sited when viewed from the ground and yet can still suffer a significant portion of its expected output due to shading that had not been taken into account—for example, the shadow of a neighbouring building sweeping across a corner of the roof for two hours each winter afternoon, or the shadow cast by a chimney at 9 am in December being longer than anyone who examined the roof at noon in June would ever have noticed. The losses caused by shading on solar panels are generally not dramatic or obvious; they are a gradual and continuous deduction from production which only becomes apparent months later when the energy bill is lower than expected.
Why This Is Genuinely Hard to Judge by Eye
The difficulty isn't that shading is hard to see; it's that the amount of shading varies greatly with the time of day and the season, and almost no one examines a roof over an entire year before fitting the panels. If you visit a site at noon in July, this tells you very little regarding how the sun's angle and the length of the shadows will behave at 9 am in December, when the sun is much lower in the sky, and the shadows are several times longer. Making this mistake doesn't only result in reduced power output; it means that a homeowner or a business ends up paying for panel capacity that they are not actually able to use for a significant part of the year.
What We Built to Actually Answer This
We have created a free tool, the Solar Panel Shadow Inspector, so that this can be checked before the panels are installed rather than having to find out later on. By entering an address, the tool displays the surrounding buildings in three dimensions and shows how their shadows actually fall on a roof—at any time of day and throughout the entire year—complete with winter and summer solstice settings for the two most extreme shadow conditions that a site may ever experience. The shadow-cast mode takes this a stage further by colouring the roof surface according to the total number of hours it spends in shadow over a selected time period, turning the question of whether a particular spot is shaded into a real visual heatmap rather than making an estimate based on a single visit to the site.
What's Actually Powering the 3D Buildings
The tool is based on Esri's 3D Buildings scene layer, which integrates data provided by Maxar, TomTom, and contributors to the Esri Community Maps in order to provide building heights and geometry that are more accurate than those usually available from OpenStreetMap 3D data alone. This is particularly important for shadow simulation since a building's height has a direct effect on the length and distance of the shadow it casts—incorrect building heights would result in inaccurate shadow predictions, thereby nullifying the tool's whole purpose.
An Honest Limitation Worth Stating Plainly
The heights and shapes in this dataset are only approximate and trees are entirely excluded from the model — a significant point, since a mature tree can create a shadow just as problematic as a nearby building, and none of the existing large-scale 3D building datasets take vegetation into account. This is a real limitation, not just a small aside: a roof which appears to be free from building shadows in the tool might still be actually shaded by trees that the model cannot represent. That is the reason why we have designed the tool as a planning guide rather than as a definitive solution — it helps to identify the areas where the actual risk is most likely to be, but on-site verification before installation is still necessary, not optional.
How to Actually Use This Before Installing
The way to proceed in practice is to first search for the address, then zoom in on the roof and check both solstice settings, as these two positions show the maximum and minimum conditions that any point on the roof will face during the year—if a particular spot for the panels remains unshaded at both the winter and summer solstices, it is a good option. Next, use the daylight timeline to go through a full day at the time of year that is most important for the given installation (the winter solstice is generally the more conservative and higher-risk test since shadows are longest and the sun's daily path is shortest). Finally, use the shadow-cast mode for the entire year to obtain a comprehensive overview, and then visit the real site to look for any trees or other obstructions that the 3D model cannot detect.
Why This Matters Beyond a Single Rooftop
Traditionally, shading analysis has had to rely on either carrying out a physical visit to the site using a solar pathfinder device or using commercial software, which comes with a real cost — in both cases, the time and money involved are incurred before a homeowner or small installer has even determined whether the site is worth looking into. By making a truly useful version of the shading analysis freely available and accessible before any site visit is arranged, the obstacles to the type of early decision-making that decides whether or not a solar installation ends up producing the output for which it was sized are reduced.
What a Real Shading Loss Actually Looks Like
Shading does not reduce solar output in the manner that most people suppose. Since the panels are generally connected in series strings, a shadow that falls on just a small part of one panel will reduce the output of the whole string to which it is connected, not merely that of the shaded cell — this represents a disproportionate loss in relation to the actual area that is shaded. That is precisely why a small shadow, such as one cast by a chimney on one corner of an array for ninety minutes each day in winter, can result in a significantly greater loss of production than the shaded area alone would indicate. It is also why it is the only way to detect this type of problem by examining the shadow patterns over an entire day and season rather than at a single moment.
Frequently Asked Questions
Does this tool take the place of a professional solar site assessment?
By no means does it replace the final on-site verification; it is intended to be used as a planning and pre-screening tool, helping to identify the most probable problem areas and providing a realistic initial evaluation of a site before deciding on a full professional assessment or an installation quote.
Why does the tool choose to separate the winter and summer solstices rather than simply displaying 'today'?
The winter and summer solstices are the most extreme shadow conditions that a roof will go through over the course of a year since at the winter solstice the sun is at its lowest angle and the shadows are therefore at their longest, whereas at the summer solstice it is the opposite. By looking at both extremes, you get a realistic range covering the worst-case and best-case scenarios rather than relying on a single arbitrary moment which might not correspond to the actual conditions that are important when correctly sizing a system.
What is the right thing for a person to do when the tool indicates a potentially shaded area?
See it as an indication that further investigation is needed, not as a final conclusion. If you can, visit the site at the relevant time of year, look out specifically for any trees or other obstructions that are not shown in the 3D building model, and think about whether the arrangement of the panels or the use of micro-inverters could lessen the effect of partial shading on that part of the array.
Try It Yourself
The Solar Panel Shadow Inspector is free to use; simply enter an address and, in just a few clicks, you will be able to get a real first impression of how shadows move over that particular roof throughout the year before spending any money on a site that might not perform as it does on paper.
