Solar photovoltaics on a pitched roof: what to model

A solar pv energy statement fails review for one of two reasons: the array shown on the roof plan will not physically fit, or the yield figure has no stated basis. Both are avoidable in an afternoon. The permitted development position for a roof-mounted array on a house is comparatively generous, so the planning risk on a typical self-build or extension usually sits in the supporting calculation rather than in the principle.

Get the geometry right first, then the yield, then the presentation.

Permitted development limits for domestic arrays

Solar equipment on a dwellinghouse or a block of flats falls within Part 14 of Schedule 2 to the General Permitted Development Order. The conditions that matter on a pitched roof are short.

• Equipment must not protrude more than 200 mm beyond the plane of the roof slope, measured perpendicular to that plane.
• It must not be higher than the highest part of the roof, excluding any chimney.
• It must be sited, so far as practicable, to minimise the effect on the external appearance of the building and on the amenity of the area.
• Equipment no longer needed for microgeneration must be removed as soon as reasonably practicable.

Two designation traps follow. On a listed building, listed building consent is required whatever the permitted development position says, and roof panels on a principal elevation are frequently refused. In a conservation area or World Heritage Site, wall-mounted equipment on a wall fronting a highway is outside the right, and an Article 4 direction can remove the roof right too.

Ground-mounted arrays sit in a separate class with its own limits on height, area, number and distance from the boundary. Do not assume the roof rules transfer. And a lawful development certificate is the only formal way to confirm that works are lawful, which is worth the fee where the roof is unusual or the property is in a designated area.

Orientation, pitch and shading in the model

The three geometric inputs are azimuth, pitch and shading, and only the third is commonly done badly.

Azimuth and pitch come from the drawings, so state them as numbers rather than as "south facing". A roof at 30 degrees pitch facing 40 degrees west of south is a materially different proposition from due south, and a reviewer who has to derive that from a plan will assume the worst.

Shading is the input that separates a real model from an optimistic one. Near shading from the building's own chimney, dormers, roof lights, plant, parapets and any proposed extension has to be modelled in three dimensions, along with trees and neighbouring buildings within a distance that depends on their height. Trees are the awkward case: model them at mature height, not at planting height, and say which you used.

Where shading is unavoidable across part of the array, describe the electrical mitigation. String layout, module-level optimisers or microinverters change the output of a partly shaded array considerably, and an assessment that ignores them understates the scheme.

Array area against available roof area

Available roof area is not roof area. Subtract the setbacks the mounting system requires at eaves, verge and ridge, the clearance around roof lights and vent terminals, and any zone kept clear for access or fire service requirements where the local authority applies one.

Then check the module arithmetic. An array is a whole number of modules in a rectangular grid, so the usable area rounds down, sometimes sharply. A plan showing a tidy filled rectangle that does not correspond to a whole number of a real module size is the first thing an assessor will test.

Structural capacity is a separate question that belongs in the application file even though it is a Building Regulations matter. A roof designed for a tile load will usually accept an in-plane array, but a retrofit onto an old rafter structure, or a roof already carrying a heavy natural slate, needs a structural opinion before the energy statement assumes the full array.

Annual yield assumptions and their basis

The single most useful sentence in a solar section states the specific yield used, its units, and where it came from. Everything else is arithmetic.

State the installed capacity in kilowatts peak, derived from the module count and the module rating, and the annual generation in kilowatt hours, derived from the capacity, the specific yield and the modelled losses. Name the irradiance dataset or the standard estimation method you used, and give the location, orientation and pitch inputs so the figure can be reproduced. Then list the loss assumptions separately: shading, soiling, temperature, inverter efficiency, cable losses and degradation over the assessment period.

Where the assessment feeds a Building Regulations calculation, keep the two consistent. The generation figure in an Energy Statement should reconcile with what the compliance software has been given, and a mismatch between the planning submission and the design stage output is a question you will be asked at discharge of conditions.

Label any worked figure as illustrative where it is used to demonstrate a method rather than to represent the actual scheme.

Batteries and their treatment in the assessment

Battery storage does not generate anything, so it does not increase the array's output and should not be presented as if it does. What it changes is the proportion of generation used on site rather than exported, which matters to the occupier's bills and to any policy that measures on-site use.

Say what the battery does in your assessment and what it does not. If the policy the scheme is being tested against measures regulated carbon under the compliance methodology, storage may contribute little or nothing to the figure even though it is worth installing. Being straight about that is better than a claim a reviewer will strike out.

Siting is a planning-adjacent matter. A battery in a garage, an outbuilding or on an external wall raises fire separation and access questions that the building control body will look at, and a unit mounted on a front elevation raises appearance questions in a conservation area.

Conservation area and listed building constraints

In a conservation area, the test the officer applies is whether the proposal preserves or enhances the character and appearance of the area. Roof slopes visible from the street attract the most attention, and a rear slope solution will usually pass where a front slope one will not.

For listed buildings, work from significance rather than from policy wording. A Heritage Statement that identifies the roof's contribution to significance, tests alternatives including outbuildings and ground mounting, and proposes in-plane, low-reflectance modules with concealed fixings has a genuine prospect. One that simply asserts the panels are reversible does not.

Reversibility helps, but it is one consideration among several and it does not answer visual harm during the panels' life.

Presenting the contribution to carbon reduction

Order the presentation the way the energy hierarchy runs: demand reduction through fabric and services first, then efficient supply, then renewable generation. A solar array presented as the first move invites the question of what the fabric is doing.

Give the baseline, the regulated demand after the fabric and services measures, then the generation and the resulting reduction, with the method named at each step. Where a local policy sets a percentage reduction target, show the calculation against that policy's own metric rather than a different one. And keep the Sustainability Statement and the energy section consistent, because they are frequently read side by side.

This is general guidance rather than legal advice, and permitted development rights depend on the current wording of the Order and on any Article 4 direction, so check the position with your local planning authority 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.