Sizing a rooftop photovoltaic array for a planning submission
A planning authority checks that the roof plan could physically carry the array and that the capacity claimed matches it. Measure the usable area first, then state every assumption behind the kWp and the yield.
Photovoltaic array sizing for a planning submission is an area calculation before it is an energy calculation. The authority is checking two things: that the roof plan shows an array which could physically be built once plant, access and setbacks are taken out, and that the capacity claimed in the energy figures matches that plan. Where those two documents disagree, the officer takes the smaller number, and the carbon saving you were relying on shrinks with it.
So work from the roof outwards. Measure the area you can actually use, convert it to installed capacity with a stated assumption, and only then put a yield figure against it with a named source.
What the submission has to show
For a major application, the Energy Statement carries the calculation and the roof plan carries the evidence. Three drawings do most of the work:
- A roof plan at a scale that lets the reader count modules, with the array outline dimensioned and the unusable zones hatched and labelled.
- A section or elevation showing module height above the roof surface, any tilt frame, and the relationship to the parapet.
- A plant layout showing every competing item on the roof, drawn to the same base.
Authorities in London will also expect the array to sit within the Be Lean, Be Clean, Be Green, Be Seen sequence, with the renewable contribution quantified at the Be Green stage. Elsewhere the adopted local plan's energy policy sets the test, and the wording varies more than people expect, so read the policy before deciding what the array has to achieve.
Where the roof is doing more than one job, resolve the conflict on the drawing rather than in the text. Our note on biosolar roofs and the energy, drainage and biodiversity trade-off covers how a substrate depth and an array can be reconciled.
Usable roof area after everything else
Start with the gross roof area and subtract, in this order:
- Plant, including air source or exhaust air heat pump condensers, ventilation risers, flues, lift overruns, water tanks and any communal plant enclosure.
- Access and maintenance routes, including the route from the roof hatch or stair to every item of plant.
- A perimeter zone at the roof edge, which a flat roof needs for wind uplift reasons and for safe working, and which also keeps the array out of view from the street.
- Fall protection and anchor lines, fire service access where the design requires it, and any smoke ventilation zone.
- Areas shaded for a meaningful part of the day by parapets, cores or neighbouring buildings.
What is left is the buildable array area. On a compact residential block with a lot of plant, that can be a third of the gross roof. On a warehouse it can be most of it, which is why logistics schemes carry so much of the load in local energy policies and why our piece on energy statements for warehouse and logistics schemes treats the roof as the main event.
Two errors recur. Plant is drawn after the array, so the array is drawn over the condensers. And the substation, sprinkler tank or thermal store lands on the roof late in the design, after the energy numbers have been fixed.
Orientation, pitch and shading
On a pitched roof the modules follow the slope, so orientation is given. A south-facing slope gives the best annual yield, east and west slopes give less but a flatter daily curve that often matches residential demand better, and a north slope is rarely worth claiming.
On a flat roof you are choosing. A south-facing tilted array at a modest pitch maximises yield per module, but rows have to be spaced to stop one row shading the next, which costs area. East to west tilted arrays sit in facing pairs with almost no spacing, fit more modules on the same roof, and give more total generation despite each module producing less. Both approaches are defensible. Say which one the drawing shows and why.
Shading needs to be assessed, not asserted. A parapet upstand casts a shadow across the first row for much of the winter, and a lift overrun in the middle of a roof can take out a diagonal band. Where shading is more than trivial, the honest answer is either to move the array or to report the loss.
Converting area to installed capacity
Installed capacity is module area multiplied by the rated power density of the module you have assumed. Mainstream monocrystalline silicon modules on the market sit around a fifth of incident energy converted to electricity, which puts rated output in the region of 200 watts per square metre of module area.
From there the two roof types diverge, because the ratio of module area to roof area is what changes:
| Roof type | What limits the packing | Typical capacity per square metre of buildable roof |
|---|---|---|
| Pitched, modules in plane | Ridge, eaves and verge margins, roof windows | Close to the module power density itself |
| Flat, south tilted rows | Inter-row spacing to avoid self-shading | Roughly half to two thirds of the in-plane figure |
| Flat, east to west pairs | Minimal spacing, lower output per module | Between the two, with more modules overall |
Treat the figures above as illustrative planning-stage arithmetic rather than a design. State the module dimensions and rated output you have assumed, state the number of modules, and give the total in kWp. An officer can check that in two minutes, which is the point.
Stating the yield assumption and where it comes from
This is where energy statements lose credibility. A single annual generation figure with no method behind it invites a request for further information.
Name the method. A yield derived from the Standard Assessment Procedure, from the Microgeneration Certification Scheme calculation method for the relevant location, or from the European Commission's photovoltaic geographical information system are all acceptable, and each produces a different number for the same array. Give the location or climate zone used, the orientation and tilt, the assumed performance losses, and the resulting annual output in kWh. Then carry that number into the carbon calculation with the emissions factor the relevant version of Part L or the local policy requires.
Do not quote a generic yield per kWp from memory. Regional variation across England is real, and the figure you half-remember is probably from an older dataset with a different emissions factor attached.
Structure, access and maintenance
Three allowances belong in the planning drawings even though they are engineering matters.
Added load, because a ballasted flat roof system carries the weight of the ballast as well as the frame, and on a lightweight roof over a converted building that can be the limiting factor. Wind uplift, which drives the perimeter setback and the ballast distribution and which is calculated to the relevant Eurocode wind loading standard. And access, because an array with no route between rows will not be cleaned or repaired, and a roof with no safe edge protection will not be visited at all.
Where the building is listed, none of this is permitted development and the visual effect of the array is the deciding issue rather than the yield. In a conservation area, a slope facing a highway is treated differently from a rear slope.
Drawing the array so a condition can secure it
Authorities secure renewable provision in one of two ways, and the wording matters to you.
A condition tying the development to the approved drawings is the loose version. If the roof plan is indicative, the obligation is weak, and a later value engineering exercise can remove half the array without breaching anything.
A condition requiring the installation of photovoltaic capacity of not less than a stated figure in kWp, to be installed before first occupation and retained thereafter, is the tight version. It is also the one to aim for if you want the carbon case in the energy statement to survive to completion. Offer the number, make sure it is a number the roof can carry, and keep a margin. Committing to the theoretical maximum on a scheme that has not fixed its plant is how projects end up applying to vary the condition later.
For smaller schemes, remember that roof-mounted photovoltaic equipment on an existing building often falls within Part 14 of Schedule 2 to the General Permitted Development Order 2015, subject to limits including projection beyond the plane of the roof slope of no more than 0.2 metres and not exceeding the highest part of the roof. Listed buildings and some designated land are excluded. Where the array is a ground or canopy installation instead, our note on solar canopies over car parks sets out that route.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and have the array verified by a suitably qualified designer before you commit to a capacity figure in a condition.
Draft your Energy Statement on PlanScope in minutes, then send it for professional review.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How much roof area does one kWp of photovoltaic capacity need?
On a pitched roof with modules in the plane of the slope, roughly the module area itself, since mainstream modules produce around 200 watts per square metre of module. On a flat roof with south facing tilted rows, inter-row spacing means the same capacity occupies considerably more roof. State the module dimensions and rating you assumed.
Do I need planning permission for roof mounted solar panels?
Often not. Part 14 of Schedule 2 to the General Permitted Development Order 2015 permits solar equipment on many buildings, subject to limits including projection of no more than 0.2 metres beyond the roof plane and not exceeding the highest part of the roof. Listed buildings and some designated land are excluded.
What yield figure should I use in an energy statement?
One you can attribute. The Standard Assessment Procedure, the Microgeneration Certification Scheme method and the European Commission's photovoltaic geographical information system each give a different answer for the same array. Name the method, the location or climate zone, the tilt and orientation, and the loss assumptions, then carry the result into the carbon calculation.
Should the array be south facing or east to west on a flat roof?
Both are defensible. South facing tilted rows give the highest yield per module but need spacing to avoid self shading, which costs area. East to west pairs fit more modules on the same roof with less spacing and usually produce more in total. Say which the drawing shows and why.
How will the council secure the array in a condition?
Either by tying the permission to the approved drawings, which is weak if the roof plan is indicative, or by requiring capacity of not less than a stated figure in kWp to be installed before occupation and retained. Aim for the second, but only commit to a figure the roof can carry once plant is fixed.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.