Match Solar Panel Soiling Types to the Right Cleaning for Councils

Councils and asset managers: identify two soiling types, measure their output impact, and use site assessments to choose water first or robotic cleaning...

Solar panel soiling falls into two operational groups: aerosol deposits (dust, salt, pollen and pollution particles) and macroscopic or biological deposits (bird droppings, leaves, moss and biofilms). Aerosol layers tend to build gradually and often respond to rain or a rinse; biological and localised deposits bond harder and usually need targeted, physical removal. The right response depends on which type dominates at your site, not a fixed calendar.

TL;DR:

  • Aerosol deposits like dust and salt tend to accumulate gradually and can often be removed with rain or rinsing, whereas biological deposits such as bird droppings bond more strongly and require targeted cleaning.
  • The dominant soiling type varies by microclimate, so inspecting panels individually and tracking deposit types over time is essential for effective maintenance planning.
  • Different materials cause varying performance losses, with mineral dust, salt, and pollen each producing distinct effects that depend on particle size and site proximity to coastlines or industrial areas.
  • Visual inspections should occur every one to six months based on risk factors, and generation data comparison helps identify unexplained declines indicating the need for cleaning.
  • A site assessment before contracting cleaning services ensures the appropriate method is chosen, favoring water-first or robotic approaches for bonded residue and hard-to-reach arrays.
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Keep Council Solar Arrays Performing
BrightView provides robotic solar panel cleaning for safer, more efficient maintenance across difficult-to-reach arrays.

Table of Contents

What are the two main solar panel soiling types?

Every soiling issue on a commercial or council-owned array falls into one of two operational buckets, according to the DLR fact sheet on soiling. Getting this classification right at the start saves a lot of wasted maintenance spend later.

Aerosol deposits are fine airborne particles: mineral dust, sea salt, pollen, soot and vehicle emissions. They settle as thin, relatively even films across the module surface.

Macroscopic and biological deposits are larger and more localised: bird droppings, fallen leaves, lichen, moss and biofilms. These sit in patches rather than sheets.

The distinction matters operationally. A thin aerosol layer scatters light fairly evenly across a panel, while a dense patch of droppings or leaf litter can completely block a handful of cells. That’s a different electrical problem, and it calls for a different fix.

  • Aerosol deposits build gradually and often respond to rain, wind, or a straightforward rinse.
  • Macroscopic deposits bond to the glass and resist rain, sometimes for months.
  • Biological growth (moss, lichen, biofilm) can etch or discolour the glass surface if left too long.
  • The dominant soiling type at a site is rarely obvious from a distance. It has to be recorded panel by panel, not assumed.

Councils managing multiple sites across different microclimates will typically see one category dominate at each location, which is exactly why a single, calendar-based cleaning schedule rarely fits every asset.

Common aerosol soiling types and their characteristics

Not all dust behaves the same way, and the MDPI review of soiling loss in photovoltaic modules points to materials like laterite, limestone and fly ash producing measurably different performance effects depending on particle size and composition.

  • Mineral dust and soil particles: range from fine silt to coarser grit, blown in from roadworks, farmland or bare ground nearby. Fine particles form the most persistent, evenly spread layers.
  • Sea salt: near coastlines, salt aerosol adheres through repeated wetting and drying cycles and leaves a residue that rain often doesn’t fully remove.
  • Pollen: seasonal pulses, often sticky once mixed with morning dew or light rain, which helps it grip the glass rather than wash off.
  • Soot and vehicle or industrial emissions: fine particles that can mix with moisture and set into a bonded film rather than sitting as loose dust, per the DLR fact sheet.

Pro Tip: If your site sits within a few kilometres of a coast, main road or industrial estate, log which aerosol type shows up after each inspection. Two sites ten kilometres apart can have completely different dominant soiling mechanisms.

Why do bird droppings and organic debris cause bigger losses?

Localised deposits punch above their weight. A single patch of droppings covering a few cells can drag down the output of an entire string, because panels are wired in series and one badly shaded cell limits the whole chain, a point the DLR fact sheet makes explicit when it recommends inspecting at panel and row level rather than judging soiling from the ground.

Watch for these signs during routine walkthroughs:

  • Concentrated dark or white patches (droppings), usually near roosting spots or overhead wires.
  • Leaf litter collecting along the lower edge of tilted panels or in racking gaps.
  • Green or black films at panel edges, a sign of moss, lichen or biofilm taking hold in shaded or damp corners.
  • Streaking that persists after rain rather than washing away, often indicating a partially bonded residue underneath fresh dust.

Row-by-row inspection with dated photos gives you a record you can compare month to month. For portfolios spread across multiple councils or sites, that photographic log becomes the evidence base for deciding where cleaning is actually justified.

How does soiling type affect energy output?

Soiling reduces output two different ways. Macroscopic deposits like droppings or leaves cause hard shading, physically blocking light from reaching specific cells. Aerosol films cause soft shading, scattering and absorbing light across the whole surface rather than blocking it outright. Hard shading tends to cause sharper, more concentrated losses per affected cell; soft shading spreads a smaller loss across a much larger area.

Hard and soft solar panel shading comparison

Material composition changes the scale of that soft-shading loss. Research reviewed by MDPI links deposition density directly to measurable power loss, with red soil and limestone studies showing that even small increases in deposited mass produce a detectable drop in output. Laterite, limestone and fly ash each behave differently on glass, which is why a generic “dust equals X% loss” figure doesn’t transfer cleanly between sites.

System design also changes sensitivity. Panels mounted at a steeper tilt shed loose aerosol more readily than near-flat installations. Tracker-mounted arrays that shift angle through the day can outperform fixed-tilt systems in dusty regions simply because they don’t sit at the same low angle collecting dust hour after hour.

How do I inspect and monitor for soiling?

Energy.gov.au’s guidance on looking after your solar system recommends servicing by a qualified person roughly every two years, adjusted for your system’s own documentation and site conditions. Between formal services, a lighter internal check keeps you ahead of problems.

  1. Walk the array visually every one to three months in high-risk sites (coastal, dusty, near vegetation), and every four to six months in lower-risk urban sites.
  2. Compare generation data against expected output for the season. A gap that isn’t explained by weather is your first flag.
  3. Check individual strings or rows, not just the array total. A one-string drop can hide inside otherwise healthy array-level numbers.
  4. Inspect after storms and after the first rain following a dry spell. Light rain can leave streaks or slick mud that actually increase how well the next dust layer sticks.
  5. Log every finding with a date, photo and soiling type. That record is what a contractor or council auditor will want to see later.

Pro Tip: Don’t treat “it rained recently” as confirmation the panels are clean. Check for streaking and patchy residue first; rain sometimes just redistributes the problem.

Which cleaning method suits each soiling type?

There’s no single fix for every soiling type, and the IET review of soiling accumulation and removal methods sets out the trade-offs plainly.

  • Natural removal (rain and wind) works reasonably well on light, loose aerosol dust on steeply tilted panels. It’s unreliable for salt residue, bonded pollution films or anything already partly stuck.
  • Manual and spray cleaning handle bonded or localised residue, droppings and leaf debris that rain leaves behind. A water-first approach, using minimal or no chemical additives, protects both the glass coating and the surrounding site, particularly on solar farms with sensitive land use conditions.
  • Robotic and automated cleaning suits large arrays or sites where manual access is difficult or unsafe, cutting labour risk and often water use per panel compared with manual washing.
  • Anti-soiling coatings show promise in reducing adhesion but their durability varies by product and climate exposure.
  • Electrostatic and electrodynamic systems remain largely experimental, with solid laboratory results but limited long-term field validation.

Water availability, safe access and environmental compliance usually decide which method actually fits your site, more than which method performs best on paper.

What should councils check before hiring a cleaning contractor?

Height and electrical risk make solar panel cleaning a job for qualified people, not a DIY task, particularly on rooftop or ground-mount arrays with limited safe access. Energy.gov.au’s guidance backs this directly, recommending qualified servicing over ad hoc cleaning.

Before signing a contract, confirm:

  • The scope covers inspection, cleaning and a basic electrical check, not cleaning alone.
  • Water sourcing and runoff disposal are addressed, especially on sites with stormwater or environmental conditions attached.
  • The contractor carries appropriate insurance and can demonstrate compliance with relevant workplace safety obligations.
  • Soiling-type records from previous inspections are built into the scope, so the contractor isn’t cleaning blind.
  • A pilot or trial period is available for larger or higher-risk sites before committing to a full contract.

Publisher perspective: BrightView’s practical approach to soiling and maintenance

Most soiling advice defaults to a chemical wash on a fixed schedule. We think that gets it backwards. A water-first cleaning approach and robotic access can effectively address bonded residue, hard-to-reach arrays and sensitive vegetated sites, which often don’t respond well to a one-size approach. A proper site assessment, run before any pilot programme, tells you which soiling type actually dominates and what response it deserves.

— BrightView

Some providers focus on matching the right response to the soiling type actually present at a site, rather than relying on a calendar-based approach. For asset managers and councils running arrays across different microclimates, that starts with a proper site assessment to classify the dominant soiling mechanism before any cleaning contract is signed.

Brightview Solar & Groundskeeping

From there, BrightView’s robotic solar panel cleaning service handles bonded residue and difficult-access arrays with a water-first, chemical-minimising method, reducing both labour risk and water use compared with manual crews. Where leaf litter or vegetation growth is the real driver of soiling, our vegetation management under solar arrays service tackles the cause rather than just the symptom. For larger portfolios or higher-risk sites, we run structured pilot programmes so you can validate the approach on one site before rolling it across a portfolio. Managing solar assets across multiple locations benefits from a site assessment to determine which soiling type is driving maintenance costs.

Sources

FAQ

What are the main types of solar panel soiling?

Soiling splits into two operational groups: aerosol deposits like mineral dust, sea salt, pollen and pollution, and macroscopic or biological deposits like bird droppings, leaves, lichen and moss, as outlined in the DLR fact sheet. Which type dominates depends entirely on your site’s location and surroundings.

Does rain clean solar panels effectively?

Rain often clears light, loose aerosol dust on steeply tilted panels, but it frequently fails on bonded pollution films, salt residue or dried droppings. Light rain can even leave streaks that make the next dust layer stick harder, so check panels after rain rather than assuming they’re clean.

How often should solar panels be inspected for soiling?

Energy.gov.au suggests a professional service roughly every two years, adjusted for your system’s own documentation, alongside visual checks every one to six months depending on site risk factors like coastal exposure or nearby dust sources. High-risk sites near coastlines, farmland or industrial areas warrant more frequent checks.

Can robotic cleaning handle all soiling types?

Robotic cleaning suits large arrays and difficult-access sites well, particularly for bonded aerosol films and general surface residue, and it typically reduces water use and labour risk compared with manual washing. Heavily localised deposits like fresh droppings sometimes still need a targeted first pass before broader cleaning runs.

When should an asset manager call a professional cleaning service?

Call in a qualified service when residue persists after rain, generation data shows an unexplained drop, or a storm or seasonal change leaves visible buildup across multiple rows. BrightView’s robotic solar panel cleaning service is one option worth assessing for sites with difficult access or repeated soiling issues.

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