Does rain clean solar panels?

The short answer

Rain helps a little, but it is not a substitute for cleaning. Heavy rain rinses off some loose dust from well-tilted panels. Light rain often makes things worse. And rain does almost nothing to bird droppings, salt film or dirt that has baked onto the glass in the sun.

In Malta and Gozo there is a bigger problem: for most of the summer there is no rain at all.

What rain removes, and what it leaves behind

Rain can move:

• Loose dust that has not yet stuck to the glass.
• Light debris such as leaves and pollen.
• Some of a fresh salt film, if the rain is heavy enough.

Rain leaves behind:

• Bird droppings, which dry hard and stick to the glass.
• Baked-on dust, which has been heated on the panel day after day.
• Red rain residue, which dries into a gritty reddish layer.
• Mineral spots where water has dried on the glass.
• A dirty band along the bottom edge of the panel, where water collects before it drains.

Why light rain leaves panels dirtier

A heavy downpour has enough water to carry dust off the panel. A light shower does not. It wets the dust, the drops run a short way and stop, and then the sun dries them. The dust ends up gathered into spots and streaks, and a thin muddy film that is harder to rinse off than the dry dust was.

That is why many owners notice their output does not recover after a light shower, and sometimes drops.

Malta’s dry summer problem

Malta has a long, dry summer, usually with little or no meaningful rain for months. Through that time:

• Dust keeps building up from fields, roads, quarries and building sites.
• Salt keeps landing on roofs near the coast.
• Birds keep perching on raised frames and parapets.

And it all happens during the months when the sun is strongest and panels should be producing the most. By the end of summer, many panels carry several months of dirt.

The first autumn rain

The first showers of autumn are often light and dusty. Instead of washing the summer’s dirt away, they turn it into a muddy layer that then dries hard in the still-strong September sun. If you have been waiting for the rain to clean your panels, this is usually when you find out it will not.

Red rain and Saharan dust

A few times a year, the xlokk brings fine dust from the Sahara. When it rains during or after one of these episodes, the rain itself is carrying the dust. It dries into a reddish, gritty film on panels, windows and cars.

This is rain making panels dirtier, not cleaner. Wait for the episode to pass, then clean. Our guide to red rain and Saharan dust explains it in more detail.

Why flat roofs make it worse

Most Maltese homes have flat roofs, so panels usually sit on frames with a fairly shallow tilt. That matters:

• Steeper panels shed rain faster, and the water carries more dirt with it.
• Shallow panels drain slowly. Dirty water sits on the glass and dries there.
• The bottom edge of a shallow panel collects a line of dirt, where water pools against the frame before it runs off.

So the same shower that partly rinses a steep panel elsewhere can leave a Maltese rooftop panel looking worse.

How to tell if rain has done the job

You do not need to climb onto the roof:

• Look from a safe spot, such as a higher window or the roof door, or take a photo with your phone’s zoom.
• Check your inverter app. Compare the daily total on a clear day after the rain with a similar clear day before it. If production has not recovered, the rain did not clean the panels.

Our guide on whether cleaning solar panels increases output explains how to compare readings properly.

When to clean in Malta

• At the end of the dry summer, usually September or October.
• After a red rain episode, once the haze has cleared.
• In spring, before the longest days.

For most homes, once or twice a year is enough. Our seasonal home maintenance calendar shows where it fits in the year. Seafront roofs, roofs near building sites or fields, and roofs with a bird problem may need more. See our guide on how often to clean solar panels in Malta.

When to call a professional

Rain will not shift baked-on dirt or droppings, and cleaning panels yourself on a flat roof with low parapets and raised frames is not always safe. A professional clean makes sense for panels on raised frames or over a roof washroom, for larger systems, and for heavy dirt. Purified water is used so the glass dries without spots.

clear.mt arranges solar panel cleaning across Malta and Gozo. Tell us roughly how many panels you have and send a photo of the roof for a free quote.