Solar Panel Orientation and Shading

Orientation and shading together decide how much light actually reaches each panel, and they matter more than roof size alone. Azimuth, the compass direction a roof faces, sets when production peaks: a south-facing roof in the northern hemisphere peaks around midday, an east-facing roof peaks in the morning and a west-facing roof peaks in the afternoon, while pitch, the roof slope, sets how directly sunlight strikes the panels through the seasons. A steep roof catches low winter sun better, a shallow roof catches high summer sun better, and neither direction nor pitch can be judged without the other, since a south-facing roof at a very shallow pitch and a west-facing roof at an ideal pitch can produce surprisingly similar annual totals in different patterns.

Shading then subtracts from that theoretical pattern, and it does so unevenly. A chimney, a dormer, a tree or a neighbouring building can shade only part of an array for only part of the day, yet on a string inverter design that partial shade can reduce the whole string output, while on a design with optimisers or microinverters the loss is confined to the shaded panels. Shading also changes with seasons, as trees leaf out, as sun angle falls in winter and as new buildings or extensions appear, so a shading survey done in winter can miss summer shade and vice versa.

A good layout responds to the actual shading map, not to a symmetrical drawing. Panels that would sit in a chimney shadow for much of the day are often better moved, omitted or placed on a separate string, even if the layout looks less tidy, and panels on two different roof directions should usually be on separate strings or use panel-level electronics, since they peak at different times and a single string forces them to a compromise operating point.

Pitch and orientation also interact with household use. A household that is home in the morning may benefit more from an east-facing array, a household that cooks and runs appliances in the evening may benefit more from a west-facing array, and a household with a battery may benefit from a spread across east and west that lengthens the production day, even if the peak is lower than a pure south array. Annual total is therefore not the only figure that matters; the daily and seasonal pattern, matched to use, is equally important.

Walk the property at different times of day and, if possible, in different seasons, noting chimneys, vents, dormers, trees, poles and neighbouring buildings, and photograph shadows on the roof itself rather than relying on a satellite image alone. Ask for a shading analysis that shows, month by month or at least seasonally, which panels are shaded and when, and for a production estimate that uses that analysis, with the string layout and inverter type stated.

Also check whether trees are likely to grow, whether a neighbour extension is planned, and whether any panel position would be difficult to access for cleaning or repair. A layout that ignores future shading growth will underperform gradually, and that slow decline is easy to miss without monitoring.

In the EU, solar orientation and shading assessment follow national building and installer standards, with member states differing on which shading tools and datasets are recognised for supported schemes and on how production estimates must be documented.

In the UK, MCS standards require shading assessment for certified installations, and production estimates must follow a recognised method, with the shading assumptions stated. Even with those safeguards, the homeowner should read the shading map itself, since a compliant method can still use an optimistic assumption, such as a tree that is assumed pruned when it is not.

A common mistake is filling every square metre of roof, including shaded corners, to maximise peak kilowatts, then losing a disproportionate share of output to shade on a string design. Another is choosing a layout for symmetry rather than for shading, so a chimney shadow falls across the middle of a string every afternoon, when moving two panels would have avoided it.

A third mistake is ignoring seasonal change: a roof that is unshaded in winter, when trees are bare, can be heavily shaded in summer, when leaves are out and production should be highest. A shading survey that covers only one season, or only midday, misses the pattern that actually determines annual output.

Keep the shading analysis, the panel layout drawing with string design, the production estimate with its shading assumptions, and photographs of the roof and shading objects. Those papers let a later installer or buyer understand why panels sit where they do, and whether a new tree or extension changes the picture.

Ask a qualified installer: how was shading measured and over what seasons, which panels are shaded and when, how does the string design limit shade losses, and why was this layout chosen over a tidier alternative. Shading-specific answers show a layout designed for this roof.