Overshading and solar access for photovoltaic arrays

Solar access is the input most often overstated in an energy strategy. A photovoltaic array sized on unshaded irradiance and then installed behind a parapet, a lift overrun and a line of semi-mature limes will miss its predicted output, and the shortfall shows up years later when someone checks the metered output against the energy statement. Model the shading before the array area is fixed, using the real horizon and the real roof obstructions, and state the shading assumptions in the document so the officer is approving a yield that has been derated rather than one taken from a datasheet.

Why shading losses get understated

Two habits cause it. The first is modelling the array as an area rather than as strings, which hides the effect of partial shade on a single module. The second is treating the surrounding context as it exists on survey day and never asking what it will look like in fifteen years.

Partial shade behaves disproportionately. A module in a series string that loses part of its cell area pulls down the current available to the whole string, so a chimney shadow crossing two modules at ten in the morning costs far more than its share of the roof area. Bypass diodes limit the damage without removing it.

The second habit is the one that gets written into planning documents. An energy statement that claims a generation figure without stating the shading factor applied is giving the authority a number it cannot check.

Modelling shading from buildings and trees

Use a horizon profile taken from the array position, not from ground level at the site entrance. On a flat roof that means the parapet, plant enclosures, lift overruns, flues, aerials, and the neighbouring building that is taller than yours on the south side.

The SAP methodology bands overshading into categories, from none or very little through modest and significant to heavy, each with a multiplier applied to the predicted output. Those bands are blunt but they are the common currency for residential compliance work, so if you model in more detail, report the equivalent band as well.

For anything above a handful of kilowatts peak, run an hourly simulation with the obstructions modelled geometrically. Report three things: annual yield after shading, the percentage reduction attributable to shading alone, and the worst-affected string. That last figure is what tells the client whether to rearrange the layout or accept the loss.

Self-shading and inter-row spacing

On a flat roof the array shades itself, and the trade-off between row spacing and installed capacity is the design decision that matters most.

Tilt the modules steeply and each row casts a longer shadow, so rows move apart and fewer fit. Lay them shallow and more fit, at the cost of per-module output and with more soiling because rain runs off less effectively. East-west butterfly arrangements abandon the optimal southerly orientation deliberately, accept a lower yield per kilowatt peak, and often produce more total generation from a given roof because the packing density is so much higher.

Set the spacing from the solar altitude at a stated design condition rather than from a rule of thumb. Many designers use the altitude at mid-morning and mid-afternoon on the winter solstice, accepting some shading either side of it, and the point is to state which condition you used so the calculation can be followed. Structural and wind loading will constrain the answer too, particularly on a ballasted system, so the layout is rarely a pure solar optimisation. Rooftop plant enclosures interact with both, as covered in wind effects of a rooftop plant enclosure.

Future development on neighbouring land

There is no planning protection for solar access to a photovoltaic array, and no private law right of light that assists either, because rights of light relate to light through windows for ordinary use of a building rather than to irradiance on a generator. If the adjoining owner builds upwards lawfully, your yield falls and you have no remedy.

That has three practical consequences.

1. Check the neighbouring sites for allocations, extant permissions, prior approvals for upward extension and any development brief in an area action plan.
2. Model the yield twice, once against the existing context and once against a plausible future massing based on those documents, and report both.
3. Where the difference is material, put the array where a taller neighbour cannot reach it, usually the centre or north side of a large flat roof rather than the southern edge.

An officer reading a sensitivity case like that is more likely to accept the headline figure, because you have shown the number is not fragile.

Trees, growth and the maintenance question

Trees complicate solar strategies because their shading increases and because you usually cannot remove them. A tree protected by a Tree Preservation Order or standing in a conservation area is protected regardless of the effect on a solar array, and improving photovoltaic yield is not a reason an authority will accept for consent to fell or reduce. Even routine crown reduction needs the authority's consent for a protected tree, and works in a conservation area need six weeks' written notice.

So model the canopy at mature spread rather than today's spread, and take the array away from the tree rather than assuming the tree will move. Where a scheme depends on retained trees for landscape or biodiversity reasons, that constraint is fixed and the solar layout has to accommodate it.

Deciduous species help in one respect. Winter shading through bare branches transmits more light than summer shading through full canopy, and winter is when the generation is lowest anyway, so the annual loss from a deciduous tree to the south is smaller than a summer site visit suggests.

Optimisers, micro-inverters and their limits

Module-level power electronics reduce mismatch losses by letting each module operate at its own maximum power point, so a shaded module no longer drags its string down. They recover part of the loss.

They do not create irradiance. A module in shadow generates according to the light reaching it, and no electronics change that. Treat optimisers as a way of protecting the unshaded modules rather than as mitigation for a badly placed array, and be careful about claiming a percentage recovery in a planning document unless the figure comes from the simulation you actually ran.

Evidencing the yield claim in the energy statement

The Energy Statement has to let a reviewer reproduce the figure. Name the simulation tool and version, the module and inverter assumed, the tilt and azimuth, the horizon profile source, the shading factor or band applied, the soiling and degradation assumptions, and the resulting annual kilowatt hours with the carbon saving derived from the emission factors you have cited.

Then state what the figure depends on. Where the yield assumes a neighbouring building stays at its current height, say so. Where it assumes a tree is managed at its current size, say so and name who is responsible. Conditions requiring the installed capacity to be delivered as approved are common, and the way they are drafted matters, as set out in securing an energy strategy through planning conditions. Domestic and small commercial arrays may not need permission at all, which is covered in flat roof photovoltaics and permitted development limits.

PlanScope drafts an Energy Statement structured to the sections a local planning authority expects and flags the site-specific figures it cannot verify instead of inventing them, which is the right behaviour for a yield that depends on a shading model. For specialist technical reports, qualified professional input may be required.

Enter the site postcode on PlanScope to see which energy and sustainability documents the scheme is likely to need.

This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.