A SHORT SHOWER DOES NOT CLEAN PANELS. IT CAN FIX THE DUST IN PLACE.
Manufacturers acknowledge that rain has a self-cleaning effect on sufficiently tilted panels. The problem comes after long dry spells: a short summer shower turns dust into a thin mud spread across the entire glass surface. Under the sun that follows, the water evaporates within minutes and the remaining deposit adheres to the anti-reflective coating.
On low-tilt industrial rooftops the effect is more pronounced: water and dirt remain on the lower edge of the panels, where uneven shading can cause abnormal heating. The effect is amplified by rain carrying Saharan dust.
A short shower after a long dry spell can fix the dust onto the panel instead of washing it away.
The worst time of year coincides exactly with the best: long intervals without significant rainfall occur in July and August, the months of maximum production.
Rain is also an unreliable cleaner for organic and long-standing deposits. In Madrid, researchers observed that a light shower did not wash pollen away but dragged it down and accumulated it along the lower edge of the panel. In Portugal, the soiling rate exceeded 4% per month during the spring pollen season. In Switzerland, even with more than 1,000 mm of rain a year, a persistent soiling strip formed along the lower edge of the frame, with output losses of up to 10%.
Bird droppings, oily residues from traffic and industry, and the layer that hardens onto the glass through months of repeated dew-and-dry cycles are not fully removed by rain either. This is why the IEA-PVPS soiling report (International Energy Agency Photovoltaic Power Systems Programme) recommends periodic cleaning even in regions with frequent rainfall.
Sources: Sanz Saiz et al., Applied Sciences 10(14):4733, 2020; Conceição et al., Energies 11(2):294, 2018; IEA-PVPS T13-21:2022.
WASHING WITH TAP WATER MAKES THE SITUATION WORSE
Washing with tap water makes the situation worse because mains water contains calcium, magnesium and other dissolved salts. When the water evaporates from the glass, these minerals remain as a fine white film — the effect known in the industry as “liquid chalk”.
The film has two consequences. First, it reduces light transmission, just like the dust you have just washed away. Second, and more seriously, the surface becomes rougher at a microscopic scale and retains more dust than before. The panel becomes dirty faster after washing than it did before.
Washing with a garden hose can leave the panel in a worse condition than you found it.
For this reason, professional washing uses demineralised water measured at 0 ppm TDS — water with nothing in it that can remain on the glass. It is the only method that requires no subsequent wiping.
A SINGLE SHADOW CAN TAKE ONE THIRD OF A PANEL OUT OF OPERATION
A single shadow can take one third of a panel out of operation because output does not fall in proportion to the shaded area. The cells are connected in series, and one shaded cell limits the current of the entire string — which is why panels have bypass diodes.
The diode does its job: it protects the cell, but does so by taking the entire sub-string to which it belongs completely out of circuit. In a typical panel with three diodes, this means that one shaded cell eliminates one third of the panel's production.
It does not matter what percentage of the panel is shaded. What matters is whether it is shaded.
A leaf stuck to the glass, a bird dropping, the shadow of a chimney or an aerial in the early morning — all produce the same disproportionate effect. And the shaded cell does not merely stop producing: it becomes a consumer and heats up, creating a “hotspot” that irreversibly degrades the panel and presents a fire risk.
A HOT PANEL MUST NOT BE WASHED
A hot panel must not be washed because sudden contact with cold water can propagate existing microcracks. At maximum irradiance, a panel surface reaches 45–65 °C. The contact causes the glass's surface layer to contract rapidly, generating tensile stress. Glass withstands compression well but tension poorly.
Manufacturers are explicit on this point. First Solar imposes a maximum difference of 20 °C between the water and the module. LONGi specifies an even narrower range — a maximum of 10 °C — and prohibits washing when the ambient temperature is below 5 °C.
Washing at midday, in full sun, is the most common maintenance error — and one of the most expensive.
The correct interval is early morning, at low irradiance, when the glass is cool. It also has a secondary advantage: the solar park's production is then close to zero, so the intervention does not affect the day's output.
INCORRECT WASHING CAN VOID YOUR WARRANTY
Incorrect washing can void your warranty because the Operation and Maintenance manuals issued by Tier-1 manufacturers — Jinko, Trina Solar and others — contain explicit limits on the hardness of cleaning water, the types of agents permitted and prohibited methods.
Washing with hard water, which is common in Romania, or using detergents that do not appear on the manufacturer's approved list may constitute grounds for rejecting a performance warranty claim. This is not a theoretical clause: when you seek compensation for a module that no longer reaches its guaranteed power, the manufacturer has the right to ask for proof of how it was maintained.
The question is not whether you washed the panels, but whether you can prove how.
REC, Meyer Burger and Heckert Solar explicitly prohibit pressure washing. For their modules, any intervention requires prior written approval.
BRUSHING WEARS THE ANTI-REFLECTIVE COATING
The anti-reflective coating on a modern panel's glass is a few hundred nanometres thick, usually porous silicon dioxide. It increases light transmission by approximately 2–3%. It is not a film and it cannot be restored.
Abrasion tests show that repeated brushing wears this coating, and the degree of wear depends on the coating quality and the method. The risk increases when the brush slides over dust already deposited. Some manufacturers are strict: for anti-reflective coated modules, First Solar prohibits wet contact cleaning and dry cleaning with bristle brushes.
The anti-reflective coating cannot be restored. Every mechanical contact counts.
This is why correct cleaning involves no mechanical contact with the glass: the water dissolves and carries away the deposit without anything solid touching the surface.