Trees complicate a solar site in a way that gets worse each year

A survey photographs a moving target

The shading assessment that precedes an installation records the horizon as it stands on the day someone stood on the roof with an instrument. Buildings stay where they are, hills certainly do, and a survey against those obstructions remains accurate for as long as the array lasts. A tree is a different kind of object, because it is still under construction.

This matters more than it sounds, since the modelled annual yield in a quotation is calculated from that snapshot. A design that clears a nearby canopy with a few degrees to spare is not a design with margin. It is a design that will lose that margin on a schedule set by the tree rather than by anybody involved in the installation.

Nobody in the transaction has an obvious incentive to raise this, which is reason enough to raise it yourself.

Height and distance interact in an unhelpful way

The angle a tree subtends from the array depends on its height above the panels divided by its horizontal distance from them, so growth at the top of a nearby tree translates into a steep change in that angle. A tree at the far end of a garden can gain several metres before it touches anything. The same growth on a specimen close to the house crosses the array within a few seasons.

Species is the variable that decides the timescale, and it varies by more than most people expect. Some ornamental trees are effectively finished at a modest height and will never be a problem. Fast-growing conifers planted as a screen are a different proposition entirely, and a screen planted for privacy is by definition close to a boundary and often close to a roof.

It is worth knowing what is actually growing around the building. A local arborist can tell you the mature height of a species in an afternoon, and that number is far more useful for a twenty-five year decision than another yield estimate.

Leaves come and go, and the timing is unkind

A deciduous tree shades heavily through summer and lets a surprising amount of light through once the leaves are down, which sounds like the ideal arrangement for a solar array. Partly it is. But bare branches still intercept a meaningful share of light, and a dense winter twig structure casts a broken, moving shadow that an array handles poorly.

The seasonal pattern also fails to line up with the sun. Leaf cover is at its maximum through the months of highest generation, so the loss falls precisely where the energy is, and the clear months are the ones where the sun is low and the day short. Deciduous cover is better than evergreen, and it is not the free pass it appears to be.

Evergreen shading at least has the virtue of honesty. It shows up in a survey taken at any time of year, whereas a survey conducted in February against a bare canopy will understate what happens in July.

Growth is not the only thing a tree does to an array

Falling leaves collect along the lower edge of modules and in the gap between panel and roof, where they hold moisture against the covering and block the airflow that keeps the array cool. Debris in a valley or a gutter beneath an array is harder to clear than debris in an open gutter, because the panels are in the way.

Sap, resin and bird droppings from a perch above the roof are more stubborn than ordinary dust and less likely to be removed by rain. A tree overhanging an array converts routine soiling into a recurring maintenance item, which is a running cost rather than a one-off loss.

And there is the branch itself. Limb failure over a glass-covered roof is an unlikely event with an expensive outcome, and it is a reason to look at the condition of overhanging trees rather than only at their shadow.

The options, and what each one really costs

Moving the array is the cheapest response and the one most often overlooked. If the shaded portion of the roof is the part that will deteriorate, a smaller array placed clear of the canopy may generate more over its life than a larger one that spends decades in partial shade. That is a design conversation to have before anything is ordered.

Pruning buys time and has to be repeated, which makes it a commitment rather than a solution. Many trees are protected by local rules, some are not yours to touch, and a neighbour under no obligation to consider your generation is entitled to grow whatever they like. Removal is permanent, frequently regulated, and worth thinking about on its own merits rather than as an energy decision.

Sometimes the right answer is to accept the loss and design around it, using per-module electronics so that the shaded section does not drag the rest down, and treating the affected panels as a reduced contribution rather than a fault. Occasionally the right answer is not to put an array on that roof at all. A publication that never reaches that conclusion is not being straight with you.