Shading analysis

Turning chimney, tree, dormer and horizon shade into an auditable PV yield and electrical design.

Shading analysis estimates how objects between the sun and a PV array reduce generation. It should identify both the annual energy effect and the electrical pattern: which modules are shaded, at what time and how they share strings or MPPTs.

The relevant obstacles include chimneys, dormers, aerials, parapets, trees, neighbouring buildings and the distant horizon. A photograph taken at noon in summer is not enough because the sun’s path and shadow length change by hour and season.

The MCS shade factor

MIS 3002 uses a shade factor in its standard annual-generation estimate:

annual AC output = array kWp × Kk × shade factor

Where there is an obviously clear horizon and no near or far shading, a factor of 1.00 can be used. Otherwise a suitable method must estimate the loss. MIS 3002 allows an alternative methodology only where it can be shown to be equivalent to or better than the MCS approach.

With modelling software, the factor can be derived as:

shade factor = annual generation with shade ÷ annual generation without shade

The calculation should use the same array and model in both runs, changing only the shading obstacles.

Near shade and horizon shade behave differently

Near objects can cast sharp shadows across only part of a module or string. Their position relative to the exact module layout matters. A chimney may affect one column of modules for a short period, while a parapet can shade the bottom row repeatedly.

Far objects and the horizon block lower-angle sun across a wider part of the array. Their effect is strongly seasonal and can be more uniform.

Record obstacle position and height from a representative array point. If the array covers several roof planes or a large area, one observation point may not describe them all.

Do not climb onto a roof merely to obtain a reading without suitable access and work-at-height controls. In many cases measurements, plans, photographs, drone survey or safely positioned instruments can provide the evidence.

Electrical loss is not always proportional to shaded area

Modules in a series string carry the same current. Shade on part of one module can therefore alter the operating point of more than the shaded cells. Bypass diodes can route current around cell groups, limiting reverse stress but sacrificing the output of the bypassed section.

The result depends on:

module cell and bypass-diode layout
portrait or landscape orientation
which modules share a string
tracker allocation and parallel strings
inverter shade-search behaviour
use of optimisers or microinverters
the timing and shape of the shadow

Do not estimate string loss by multiplying percentage shaded area by annual generation. Model the electrical arrangement or use a method validated for it.

Design shade out before adding electronics

The first options are often physical:

move modules away from the obstacle
leave a gap around a chimney or dormer
change portrait or landscape layout
allocate differently shaded groups to separate trackers
use another roof plane
reduce row self-shading on a flat roof

Module-level power electronics can isolate some mismatch and improve visibility, but they cannot create sunlight. Their claimed gain must be modelled against the same layout, and added roof components introduce connectors, electronics and future access needs.

Microinverters and optimisers also differ in voltage, current, rapid-shutdown, monitoring and compatibility requirements. Choose them as part of the complete system, not as a universal “shade fix”.

Trees and changing surroundings

Tree shade can change with leaf cover, growth and maintenance. The survey should state whether vegetation was in leaf and what management assumption supports the estimate. Do not count on pruning land or a tree the system owner does not control.

New construction can also change the horizon. Planning status and future rights to light are legal questions outside a yield model; the estimate can only record known site conditions and stated assumptions.

Assessment audit trail

A reproducible assessment records:

site plan or image showing shading objects
survey date and method
array observation points
monthly or annual loss by roof plane
MCS shade factor and how it was derived
string, tracker and module layout
model with and without any proposed module-level electronics
assumptions about vegetation and future obstruction
warning where the site was assessed remotely

A single percentage with no diagram or method is not an auditable shade assessment.

Checking after installation

Module-level monitoring can help identify repeatable shadow patterns, but different module readings also reflect orientation, temperature, tolerance and equipment faults. Compare like-for-like modules at the same time and review several clear days.

Unexpected new shade, persistent bypass activation or local heating should be investigated. Never use a monitoring difference alone to declare a module faulty without considering the physical shadow and electrical layout.