Sunlight to solar panels and photovoltaic yield

Overshadowing of solar panels by new development is assessed by modelled annual energy yield, not by the window-based tests that govern daylight to a neighbour's rooms. A vertical sky component figure tells you nothing useful about an array, because a photovoltaic module responds to direct beam irradiance on a tilted plane across the year, and because the electrical behaviour of a partly shaded string is not proportional to the shaded area. So the right output is a comparison of kilowatt hours before and after, per year, with the shading geometry stated.

Whether that loss carries weight in a planning decision is a separate question, and the honest answer is that it usually carries some, rarely a lot.

Why array shading is assessed differently from window daylight

BRE 209 sets targets for vertical sky component, no-sky line and annual probable sunlight hours at windows, and those tests were designed for the interior environment of habitable rooms. They measure the sky visible from a point and the proportion of probable sunlight hours retained.

A photovoltaic array cares about different things. The plane of the modules is tilted and oriented, so the relevant quantity is irradiance on that plane. Diffuse and direct components behave differently under obstruction. And the loss is concentrated in the hours and seasons when the obstructing building sits between the array and the sun, which for a building to the south means the middle of a winter day when output is low anyway.

That last point cuts both ways. A large apparent obstruction can cost surprisingly little annual yield. A smaller one due south at a low angle can cost more than the geometry suggests. Only the model tells you which.

Modelling annual yield loss rather than hours

The method is established and the outputs are easy to check.

1. Build a three-dimensional model of the array, the existing surroundings and the proposed development, on a correctly oriented site survey.
2. Use a recognised irradiance dataset for the location, with an hourly or sub-hourly time step across a full year.
3. Generate a shading profile for each part of the array, computed from the sun path for the site latitude.
4. Run the baseline case with existing surroundings and the proposed case with the new development in place.
5. Report specific yield in kilowatt hours per kilowatt peak for both cases, the absolute difference, and the percentage change.
6. Show the monthly distribution, because an annual figure hides a loss concentrated in one season.

State the assumptions: module orientation and tilt, the inverter and string configuration, whether module-level electronics are fitted, and the datasets used. An assessment that gives only a single percentage figure cannot be checked, and a consultee will say so.

String layout and partial shading effects

This is the part that separates a proper assessment from a shadow diagram.

Modules in a series string carry the same current. When one module is shaded, it limits the current for the whole string unless bypass diodes route around it, and bypass diodes typically operate at the level of a group of cells rather than a single cell. The result is that shading a small fraction of the array area can remove a disproportionate share of the output from that string.

Three design responses matter. Arranging strings so that a moving shadow crosses one string at a time rather than clipping every string. Fitting optimisers or microinverters so that each module operates independently. And leaving the shaded row out of the design altogether where the loss and the cost of mitigation both point that way.

Where an existing array is affected by a proposed development, ask the owner for the string layout before modelling. Assuming a uniform array and reporting a proportional loss understates the effect, and it is the error most often found in these assessments.

Neighbouring development and existing trees

Trees complicate the baseline. A deciduous canopy transmits a fraction of the beam in winter and almost none in summer, and it grows. A model that treats a young tree as a fixed opaque solid overstates present loss and understates future loss.

Where trees are in the baseline, model them with a stated transmissivity for each season and say what growth assumption you have used. Where the proposal involves removing trees and replacing them with a building, be explicit that the baseline is changing in two directions at once.

Existing buildings are simpler but need the same care about what counts as baseline. Where the adjoining site holds an unbuilt permission, the baseline question is the same one that arises for daylight generally, and it should be resolved the same way, with both cases reported. The comparable exercise for amenity space is set out in our note on sunlight to courtyard amenity space in a perimeter block.

Where the energy statement and the daylight report meet

The two documents are usually written by different consultants, and they contradict each other more often than they should.

The Energy Statement for your own scheme will contain an assumed photovoltaic yield feeding the regulated carbon calculation. If the site is overshadowed by an existing or consented building, that yield assumption has to reflect the shading, or the carbon reduction claimed is overstated and the assessment is unreliable.

The reverse case is the one that comes up at planning committee: a neighbour with an existing array objecting that the proposed building will cut their generation. The applicant's Daylight and Sunlight Assessment will address windows and amenity space, and it will not answer that objection unless someone asks it to. Commission the yield analysis as a short technical note and append it.

Keep the two consistent on geometry. Nothing damages a case faster than a shadow study and an energy statement using different massing.

Mitigation through array relocation

Where a loss is material and the development is otherwise acceptable, the practical responses are limited and worth knowing before the hearing.

Relocating the affected modules to an unshaded part of the roof, if there is one. Re-stringing the array and adding module-level electronics, which recovers part of the loss at modest cost. Increasing array size to restore the previous output, where roof area allows. Or a commuted payment, which is a private matter and not something planning can require.

Note the limits honestly. A planning authority cannot compel an applicant to pay a neighbour, and a condition requiring works to a third party's roof is unenforceable. What an authority can do is treat an offered mitigation package as a material consideration in the balance.

Conditions protecting future solar access

There is no right to light for solar panels. Rights of light are a private law matter concerned with light to defined apertures in buildings, and they do not attach to a rooftop array. Planning policy protects amenity, and the loss of output from an existing renewable installation is capable of being a material consideration, but it is one that is weighed rather than decisive.

Forward protection is rarer still. Some design codes and masterplans set height parameters that preserve solar access across a site, and a few authorities have used conditions restricting later upward extensions where an array is part of the approved energy strategy. Neither is common, and neither binds a neighbouring landowner.

The practical protection is design. Put the array where it has the best chance of staying unshaded, allow for the development plan's height expectations on adjoining sites, and model the consented condition of the surroundings rather than the current one. An Overshadowing Study that includes the array plane alongside the amenity areas costs very little extra at the outset.

This is general guidance, not legal advice. 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.