Why a tree's shadow keeps changing after your system is installed
The trigonometry behind the shadow itself
Shadow clearance follows the same math as any obstruction: divide the height difference between the tree and the array by the tangent of the sun's altitude angle at the lowest point of winter. A tree just 4 meters taller than the array needs roughly 15.5 meters of clearance at a 14.5-degree winter sun angle, a distance that surprises people who assume a modest height difference only casts a modest shadow.
Why trees are a fundamentally different obstruction
A chimney or vent stack is the same height on installation day and 25 years later. A tree is not. Most established shade trees grow roughly a foot a year, which means the shadow this tool calculates today keeps stretching further across the array every single year the system operates, unlike any static obstruction on this site. That growth is why a tree that barely touches the array today can cost real output a decade from now. NLR's PV shading documentation explains how shade geometry turns a small obstruction into a large output loss.
Why the compass direction of the tree line matters
South-facing trees cause the most shading impact, since the sun tracks through the southern sky for the majority of daylight hours, a factor the output loss from tree shading calculator models directly. East-side trees mainly cost morning production, and west-side trees mainly cost afternoon production, while north-side trees rarely matter at all unless branches directly overhang the roof itself.
When trimming or removal actually pays for itself
Many solar companies use roughly 10 to 15% of blocked sunlight as the rough threshold worth addressing. Below that level, a healthy tree usually isn't worth removing for the modest production gain. Above it, the lost output over a 25-year system life can add up to a meaningful amount, making a trimming or removal conversation with a neighbor or arborist worth having before the array goes in, the same forward-looking approach the seasonal tilt adjustment calculator takes toward weighing ongoing effort against ongoing benefit. The Department of Energy's solar operations and maintenance resources cover vegetation management as part of keeping a system producing.
What to do with this number next
Check the partial roof shading sizing adjustment tool to see how a specific shading percentage affects your sizing, or the roof azimuth orientation calculator if the tree line sits on a roof face you're still deciding whether to use at all. If a battery is part of the plan, the off grid panel tilt for winter sun calculator covers the same low-sun-angle challenge from a different angle.