Shading analysis for solar PV decides whether an array meets its predicted yield or disappoints the customer, and under the MCS method each counted box on the sunpath diagram removes about 1% of AC output (The Solar Design Company, 2024). A survey that misses a chimney stack or a neighbour's tree turns a signed performance estimate into a warranty conversation.
For MCS-certified installers the shading assessment is not optional paperwork. MIS 3002 folds a shade factor into the generation calculation, so an over-optimistic survey inflates the figure the customer signs against (MCS, 2026). This guide covers the sunpath method, the near-shading circle rule, mitigation with optimisers, and the shading analysis software UK installers rely on day to day.
Key Takeaways
- MCS ties generation to a shade factor: annual yield is system size in kWp multiplied by a location factor and the shade factor, and where that factor drops below 1 the standard demands extra documentation (MCS, 2026).
- Each counted box on the MCS sunpath diagram equals roughly a 1% loss of AC yield, while crosshatched boxes are excluded from the count (The Solar Design Company, 2024).
- Shading a single cell on a 400W module can remove around 150W, close to 35 to 40% of its output, far more than the shaded area suggests (8MSolar, 2026).
- Objects within 10 metres of the array centre need a near-shading circle drawn on the diagram, sized to the height of the object (The Solar Design Company, 2024).
- Power optimisers and microinverters reduce mismatch losses, but they never replace a proper site survey and honest performance estimate.
What is shading analysis for solar PV?
Shading analysis is the site-survey step that maps every object able to block sunlight from the array across the year, then converts that into a percentage yield loss. Under MIS 3002 the result feeds directly into the performance estimate the customer receives (MCS, 2026).
The work splits into two questions. First, what casts shadow on the roof, from far hills on the horizon to a nearby flue. Second, how much of the sun's yearly path each object actually blocks. Getting the second question right is where surveys go wrong, because the sun sits low in a British winter and objects that look harmless at noon in June shade the panels heavily in December. The array layout you settle on flows from this, which is why shading and roof layout are planned together.
How does the MCS sunpath shading method work?
The MCS method overlays shading objects onto a sunpath diagram divided into boxes, then counts the boxes each object covers. Every counted box represents about 1% of lost AC yield, and crosshatched boxes are ignored because the sun contributes little energy through them (The Solar Design Company, 2024).
A worked example makes it concrete. An installer surveys a semi-detached roof, sketches the neighbour's tree and a chimney, and transfers the outline onto the official 52 degree latitude sunpath foil. The shading line passes through six boxes. Under the MCS method that equals a 6% loss, so the optimum yield is multiplied by 1 minus 0.06, giving a shade factor of 0.94 (The Solar Design Company, 2024). That single number then scales the whole performance estimate.
MIS 3002 was reissued as Version 2.0 on 18 March 2026 and becomes mandatory for certified installers from 18 June 2026, so surveys need to match the current guide rather than an older revision (MCS, 2026).
Why does a little shade cause so much power loss?
Because cells sit in series, the weakest cell throttles the whole string. Shading one cell on a 400W module can cut roughly 150W, around 35 to 40% of output, even though that cell is a tiny fraction of the surface (8MSolar, 2026). The current has to squeeze through the shaded cell like water through a kinked hose.
Bypass diodes limit the damage by letting current route around a shaded cell group, but they cannot recover the lost energy and they introduce their own voltage drop (Surge PV, 2025). This is why partial shade on one module can pull down an entire string, and why the survey has to model shadows across the day and the seasons, not just at the moment the installer is on the roof.
Near shading and far shading on the survey
Far shading comes from distant features on the horizon such as hills or a treeline, recorded as a horizon line. Near shading comes from close objects, and MCS treats anything within 10 metres of the array centre with a dedicated circle on the diagram, its apex at the top of the object and its radius equal to the object's height (The Solar Design Company, 2024).
The distinction matters because near objects move their shadow fast across the roof and can wipe out a string at certain hours, while far shading trims the edges of the generating day. A chimney two metres from the panels behaves very differently from a hill two kilometres away, and the survey has to capture both to produce an honest figure.
Which shading analysis software do UK installers use?
UK installers combine an on-site optical tool with desktop software. The Solar Site Selector foil captures the horizon and near objects through a fisheye lens on the roof, then that sketch is modelled in software such as PV*SOL or an online shade report that generates a sunpath diagram from address data (OpenSolar, 2025).
When you choose shading analysis software UK installers should check it outputs an MCS-compatible sunpath diagram and shade factor, not just a generic 3D shadow render, because the certification body wants the diagram in the handover pack. For commercial solar PV design the same logic applies at larger scale, where inter-row shading between panel rows becomes a second source of loss to model.
How can installers reduce shading losses?
Once the survey quantifies the loss, the design responds. Options range from moving panels away from the worst obstruction, to zoning strings so shaded modules sit together, to module-level electronics that isolate each panel. The right choice depends on how spread the shade is and the customer's budget.
The table below sets out the common routes and their trade-offs:
- String inverter plus layout zoning: best for unshaded or lightly shaded roofs, the cheapest option, but a single shaded module still drags its whole string. A hybrid inverter with two MPPT inputs helps by separating orientations.
- Power optimisers (DC): best for partial or spread shade, giving module-level tracking, at the cost of extra components and price per panel.
- Microinverters: best for complex multi-orientation roofs, converting each module independently, but with the highest unit cost.
- Panel repositioning: best against a fixed obstruction, removing the loss at source, but usually means fewer modules or lower total yield.
Does shading change across the seasons?
Yes, and this is where a snapshot survey misleads. The sun sits far lower in a British winter than in summer, so a tree or dormer that clears the array in June can shade it for hours in December, exactly when generation is already scarce (Surge PV, 2025). The sunpath diagram captures this by mapping the whole year, not one moment.
That seasonal swing is why the MCS method uses a sunpath rather than a single shadow photo. A low winter sun rakes across the roof at a shallow angle, dragging obstruction shadows long and across more of the array. An honest estimate weights those winter losses properly instead of assuming the summer picture holds all year, which flatters the figure and disappoints the customer.
Inter-row shading on flat and ground-mount arrays
On flat roofs and ground mounts the panels shade each other. Rows tilted up to face the sun cast a shadow onto the row behind when the sun is low, so row spacing becomes a second shading calculation on top of the surrounding objects (8MSolar, 2026). Tighter spacing packs in more panels but raises inter-row losses.
The design trade-off is real estate against yield. Wider gaps cut inter-row shading but fit fewer rows on the roof, while tighter gaps do the reverse. On commercial solar PV design this spacing is modelled explicitly against the winter sun angle, because a layout optimised only for panel count can quietly lose a chunk of its winter output to its own rows.
Recording shading for the MCS performance estimate
The shade factor is not just a design input, it is a disclosure. MIS 3002 requires the shading assessment and its effect on yield to reach the customer, so the sunpath diagram belongs in the handover pack alongside the estimate (MCS, 2026). Where the shade factor is below 1, additional detail on the affected output is expected.
In practice this protects the installer as much as the customer. A documented 0.94 shade factor with a diagram behind it is a defence against a yield complaint, whereas a rounded-up estimate with no survey record is a liability. The same standard set is tightening across the board, and the wider MIS 3002 standard is worth reading in full before the June 2026 mandatory date.
Frequently Asked Questions
Does MCS require a shading assessment for every solar PV install?
Yes. MCS-certified installers must assess shading and reflect it in the performance estimate for each job, using the sunpath method in the MCS PV guide. Where shading is present and the shade factor falls below 1, MIS 3002 asks for extra documentation of the effect on output (MCS, 2026).
What counts as a shaded box on the MCS sunpath diagram?
A box counts when a shading object covers any part of it, unless it is crosshatched, in which case it is excluded because the sun delivers little energy through that part of the sky. Each counted box equals roughly 1% of AC yield loss (The Solar Design Company, 2024).
How much yield can shading realistically cost?
It varies from a percent or two for minor far shading to a third or more of a module's output when a single cell is blocked, because series cells share one current path (8MSolar, 2026). A close object shading part of the array for several winter hours can remove a meaningful slice of annual generation.
Do power optimisers remove the need for shading analysis?
No. Optimisers and microinverters cut mismatch losses between modules, but they cannot recover light that never reaches the cells, and MCS still requires the shading survey and performance estimate. They are a mitigation for unavoidable shade, not a substitute for measuring it.
What is the 10 metre rule in near shading?
Objects within 10 metres of the array centre are treated as near shading and drawn as a circle on the sunpath diagram, with the top of the object at the apex and the radius set to the object's height (The Solar Design Company, 2024). Objects beyond that distance are handled as far shading on the horizon line.
A clean shading survey is only useful if the shade factor, diagram and estimate travel together into the customer's paperwork. Tools like Reonic help installers keep that survey data attached to the proposal so the number the customer signs matches the number the roof will deliver.
