PV soiling and cleaning

How to decide whether a PV array needs cleaning and carry it out without damaging modules or creating roof and electrical risks.

PV soiling is dirt or biological material on a module that reduces the light reaching its cells. Rain can remove loose dust from a suitably pitched array, but it will not reliably clear bird droppings, lichen, sticky deposits or dirt held at a lower frame edge.

Cleaning should be based on evidence and safe access, not an automatic annual sales visit. The likely energy recovered must justify the roof, electrical and module-damage risks.

What collects on modules

The mix depends on the site and season. Sources include:

dust, pollen and traffic particles
salt near the coast
agricultural or industrial deposits
bird droppings and nesting debris
leaves and seed material
lichen, algae or other growth
snow or ice in suitable weather

Array pitch, frame shape, wind, rainfall and nearby trees all affect whether material washes away or accumulates. A low-pitch or sheltered module can behave differently from a steep exposed one on the same property.

Soiling is not one fixed loss

A uniform film can reduce current across the array. A small opaque deposit can shade part of one cell or cell group and create a more local mismatch. The energy effect depends on irradiance, string layout, bypass-diode operation and any module-level electronics.

No national annual loss percentage applies to every UK roof. A number from a dry or dusty climate is not a sound estimate for one UK roof, and a short before-and-after comparison can confuse cleaning with a change in weather.

Decide from evidence

Use several indicators together:

safe visual inspection or suitable images
repeatable dirt patterns at module edges
performance monitoring compared with irradiance or a clean reference
unexplained mismatch between similar strings or module channels
a persistent change after accounting for weather, shade and outages

Monthly kWh alone is a weak cleaning trigger because sunlight varies. A monitoring system with a stable baseline can identify a gradual or local change more reliably.

If only one module or substring appears weak, investigate electrical faults and shading as well as dirt. Cleaning will not repair a failed bypass diode, connector or cell.

Safety comes before energy recovery

Roof access is the main hazard for most domestic cleaning. Work at height needs suitable planning, equipment and competence. Long poles used from the ground can contact overhead lines, damage modules or make it impossible to control pressure.

PV DC wiring remains energised when modules are illuminated. Turning off the inverter or AC isolator does not make the roof wiring dead. Water should not be directed into damaged connectors, junction boxes, isolators or cable entries.

Do not walk or kneel on modules. Loads can crack glass or cells without an immediately obvious mark.

Follow the module instructions

The module manufacturer’s cleaning and maintenance instructions take priority. They define acceptable water, tools, detergent, pressure and temperature conditions.

General precautions include:

use soft, clean equipment that will not scratch glass or coatings
avoid abrasive pads, blades and aggressive chemicals
avoid pressure concentrated at seals, edges and junction boxes
avoid thermal shock between hot glass and cold water
prevent hard-water deposits where water quality is unsuitable
keep tools clear of cables, connectors and frames

An anti-reflective or other surface coating can be damaged by a method that looks harmless on ordinary window glass.

Bird droppings and local deposits

Do not scrape a dry deposit across the module. Soften and remove it using the approved method. A persistent opaque deposit should not be left indefinitely because local shading can reduce output and contribute to cell heating.

Repeated fouling points to a site issue. Bird exclusion may be appropriate once active nests and wildlife law have been addressed. It must preserve array ventilation, wiring and roof drainage.

Lichen and strongly attached growth

Strong growth can be difficult to remove without scratching or using an unapproved chemical. Ask the module manufacturer or a specialist for the method suitable for that glass and coating.

Do not assume complete visual removal is worth damaging the surface. A professional should document any pre-existing etching, delamination or glass damage before cleaning.

Cleaning does not replace inspection

A cleaner may notice loose cables, cracked modules, corrosion, broken tiles or failed mesh, but a cleaning visit is not automatically an electrical inspection.

If there is scorching, melted plastic, a cracked junction box, exposed conductor or an inverter insulation alarm, stop and use a competent PV electrical contractor. Do not wash a fault in the hope that it will reset.

Verify the result

Photograph the array before and after from the same safe position. Compare performance over suitable periods with weather normalisation or a comparable clean string where possible.

The aim is not to claim an instant percentage from two readings under different clouds. It is to confirm that the identified dirt was removed, no damage was introduced and any persistent underperformance has been referred for diagnosis.

When cleaning may be unnecessary

Cleaning may not be worthwhile where modules are visibly clear, output is consistent with the model and access would introduce disproportionate risk. Routine ground-level observation and monitoring can continue until evidence changes.

The correct interval can therefore be “when indicated” rather than every fixed number of months. Manufacturer, insurer or site-specific maintenance requirements can still impose a schedule and should be retained in the handover pack.