Solar Panel Cleaning in Portugal: How Dirt Reduces Performance and Cleaning Increases Saving

Solar panels are designed to be low-maintenance, but they are not maintenance-free. Over time, dust, pollen, bird droppings, salt residue, traffic film, and other debris can build up on the glass and reduce the amount of sunlight reaching the solar cells. When that happens, the system produces less electricity, and lower production means lower savings.

This is not just theory. Soiling is widely recognised as one of the main causes of solar underperformance in the real world. The IEA-PVPS estimates annual energy losses from soiling at around 3% to 5% globally, depending on location and conditions.

For homeowners in Portugal, where many systems go through long dry spells, high pollen periods, coastal residue, and occasional dust, panel cleanliness can make a real difference to annual production.

How dirt affects solar panel performance

Solar panels work by allowing sunlight to pass through the front glass and reach the photovoltaic cells underneath. When dirt accumulates on the surface, less light gets through. That lowers energy production.

In some cases, the issue is more than just a thin layer of dust. Uneven soiling, such as bird droppings or localised grime, can create imbalance across cells and modules, reducing output more than many people expect. NREL notes that soiling can materially affect production and gives examples of around 5% loss from site-specific contamination such as nearby construction dust.

The effect varies by site. In wetter regions, rain may remove part of the dirt. In drier or dirtier conditions, losses can build over time. A Europe-wide study found average annual soiling losses of 0.9% if rainfall is assumed to clean panels perfectly, but as much as 5.3% if rainfall is only partially effective.

That tells us something important: rain helps, but it is not always enough.

How cleaning helps performance

Cleaning helps by restoring light transmission through the glass. Put simply, when the dirt is removed, the panel can capture more sunlight again.

The value of cleaning depends on how dirty the panels have become and how long they have been left that way. Research shows that once dirt accumulation becomes meaningful, cleaning can recover a noticeable amount of lost output.

One study found that panels left uncleaned for three months experienced power losses of up to 13% compared with panels cleaned daily.

Another field study found that monthly cleaning increased annual energy generation by 11.15% compared with similar uncleaned panels.

A separate study in an urban environment reported a 7.4% decline in PV output after just one week due to deposited particles, and also noted that light rain did not fully solve the issue.

These are not promises that every home in Portugal will gain those exact percentages. Every site is different. But they clearly show the principle: when dirt is allowed to build up, production falls, and when that dirt is removed, performance improves.

What this means in Portugal

Portugal is an excellent country for solar, but the same sunshine that makes solar attractive can also create conditions where dirt stays on panels for longer.

This is especially relevant for homes:

    •    Near the coast
    •    Close to roads or building works
    •    In areas with pollen-heavy vegetation
    •    With birds regularly sitting above the array
    •    After long dry periods without enough rain

In these situations, cleaning can help keep the system performing closer to its true potential.

When should solar panels be cleaned?

There is no one-size-fits-all answer. Some systems may need very little intervention, while others benefit from periodic cleaning.

A good rule is to review cleaning when:

    •    Production appears lower than expected
    •    There has been a long dry spell
    •    Visible dirt or residue is present
    •    There are repeated bird-dropping issues
    •    The home is near a source of dust or pollution

The key is to make cleaning decisions based on actual site conditions and likely energy recovery, not just appearance. NREL’s guidance on soiling management also supports this more practical, site-specific approach.

The bottom line

Solar panels can lose performance when dirt builds up. In some cases the loss is small, but in others it can become significant. Real-world studies show that cleaning can recover meaningful production, sometimes by several percentage points and sometimes by much more, depending on the site.

That is why solar panel cleaning should be seen as part of smart system ownership.

Want to know whether your solar system is performing as it should?

Visit www.solviva.biz to speak to Sol Viva about solar performance, system reviews, and how to get the most from your installation.