Shading losses in a rooftop solar calculation

A rooftop photovoltaic yield in an energy statement gets challenged on two things: the area the panels are claimed to occupy, and the losses applied to the output. Both are checkable from the drawings, which is why an inflated figure is found so often. The defensible calculation starts from a measured usable area, applies orientation and pitch, deducts overshading from obstructions the design already shows, and then applies system losses as a stated performance ratio.

Everything else in the method is arithmetic. The judgement is in the area and the shading, and those two should be drawn rather than asserted.

Establishing usable roof area

Start from the roof plan and subtract, item by item, with the deductions shown on a dedicated drawing.

1. Plant: lift overruns, tank rooms, air source units, smoke ventilation equipment, flues and their clearance zones.
2. Maintenance access: walkways, hatches, the zone around roof lights, and the fall-arrest or guarding set-back from the parapet.
3. Drainage: outlets, gutters, and the areas where a build-up would impede flow.
4. Other roof uses: amenity terrace, green roof area required by a biodiversity or urban greening policy, and play space if it is at roof level.
5. Row-to-row spacing on a flat roof, which is a function of the tilt angle chosen.

That last item surprises people. On a flat roof, a steeper tilt improves yield per kilowatt but increases the spacing needed between rows to avoid self-shading, which reduces installed capacity per square metre of roof. A shallower tilt, or an east-west arrangement where modules face in opposing directions, fits more capacity on the same area at a lower yield per unit. Both are legitimate and the statement should say which was assumed.

Once the usable area is fixed, the installed capacity follows from the module efficiency. With mainstream modules, roughly five to six square metres of module area per kilowatt peak is a reasonable working figure, and a claim implying considerably better than that needs the module datasheet attached.

Orientation and pitch

A south-facing array at a moderate pitch gives the highest annual yield in the United Kingdom. Rotating away from south reduces annual output gradually at first and then more sharply, and east or west orientations lose a meaningful fraction of annual yield while spreading generation across the morning and afternoon.

The spread matters for a building with daytime demand, because exported electricity is worth less than electricity used on site. An east-west array on an office or a school can produce a better carbon and cost outcome than a south array with the same nominal output, and saying so in the statement shows the design was thought about rather than defaulted.

Pitch interacts with maintenance and wind. Ballasted systems on a flat roof need a wind uplift calculation and a ballast weight the structure can carry, and a steeper tilt increases both. That is a structural constraint on the solar design, and it should appear in the statement as a stated assumption rather than being discovered later.

Overshading from parapets, plant and neighbours

Shading losses are where the reviewer looks first, because the obstructions are visible on the submitted drawings.

Four sources, in rough order of how often they are missed. Parapets, which shade the lowest row of a roof-mounted array through the winter and at the start and end of the day all year. Plant and lift overruns on the same roof, which cast a moving shadow across the array. Taller elements of the same building, which is the dominant effect on a stepped scheme. And neighbouring buildings, including consented but unbuilt schemes, which should be modelled where a permission exists.

The method used should be named. Compliance calculations apply an overshading factor banded by the proportion of the sky obscured at the array, running from no or very little shading through to heavy shading, and that banding is coarse. A shading analysis produced from the massing model, with a sun path study and a per-row or per-string assessment, is far more defensible on a complex roof and is what a reviewer asks for when the roof has obstructions on it.

One technical point with disproportionate effect. A single shaded module can reduce the output of a whole string, so string layout and the use of module-level optimisers change the answer. Where the array is laid out to keep shaded modules on separate strings, say so, because it justifies a lower shading loss than the geometry alone would suggest.

System losses and what to include

The convention is to express the combined effect as a performance ratio applied to the irradiance-derived output. Mainstream well-designed roof arrays commonly sit somewhere between three quarters and four fifths, and a statement claiming materially above that range should explain why.

The items that make up the ratio are the ones in the table. Two are worth attention. Temperature losses depend on the mounting: a module laid flat on a membrane with no air gap runs hotter and loses more than one on a raised frame, so the mounting detail affects the yield. And soiling depends on pitch and location, with a very shallow pitch in an urban or coastal setting needing a larger allowance and a stated cleaning regime.

Do not double count. Applying a shading factor and then a separate generic loss allowance that already includes shading is a common error, and it produces a figure the reviewer can show is wrong.

Presenting the yield calculation

One table, one page. Usable area, module area, installed capacity, orientation and pitch, the irradiation source, the shading loss, the performance ratio, and the annual output in kilowatt hours. Then the carbon saving, with the emissions factor named and dated, since grid factors are revised.

Put the roof plan showing the array layout and the deductions alongside it. A reviewer who can match the array on the drawing to the capacity in the table will accept the number. A reviewer who cannot will ask for the drawing, and the request costs two weeks.

Cross-check against the Energy Statement narrative so that the renewable contribution quoted in the hierarchy section matches the output in the table, and against the Sustainability Statement if that document repeats the figure. Mismatches between the two are routine and avoidable.

Coordinating with structure and drainage

Three coordination items decide whether the array as drawn can be built.

Structural capacity for the ballast or fixing arrangement, including wind uplift, which has to be confirmed by the engineer rather than assumed. Roof drainage, because a ballasted array interrupts flow paths and the outlet positions and falls need to accommodate it. And the interaction with a green or biodiverse roof, where substrate depth, planting height and panel spacing all compete, a conflict covered in our note on biosolar roofs.

Where roof area is short, a canopy over parking is sometimes the better answer, and it raises a different set of planning questions, as our note on solar canopies over car parks explains.

Where the claimed output gets challenged

Reviewers ask the same four questions. Does the array shown on the drawing match the capacity claimed. What was deducted from the roof area and why. How was shading from the building's own taller elements and plant assessed. And what performance ratio was used.

Answer all four in the statement and the yield figure stops being contentious. Leave one out and the whole renewable contribution is treated as unsupported, which matters where the scheme's compliance with a carbon target depends on it.

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