RAIN DOES NOT CLEAN PANELS. IT CEMENTS THE DIRT IN PLACE.
Rain does not clean panels; it fixes dust onto the glass. 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 hardens onto the anti-reflective coating.
The effect is amplified by rainfall that carries Saharan dust — an increasingly frequent phenomenon in southern and western Romania. The result is not a film that washes away in the next rain, but an opaque, adherent crust that blocks solar radiation and can only be removed through professional washing.
Rain that comes after a long dry period does not clean the panel. It fixes the dirt in place.
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.
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 tall blade of grass, a bird dropping, the shadow of a chimney 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.
As of 2026, manufacturers are explicit on this point. First Solar and most O&M procedures impose a maximum difference of 20 °C between the water and the module. LONGi specifies an even narrower range. Jinko prohibits washing 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.
As of 2026, four manufacturers — Q CELLS, REC, Meyer Burger and Heckert Solar — prohibit pressure washing altogether. For their modules, any intervention requires prior written approval.
A BRUSH SCRATCHES THE ANTI-REFLECTIVE COATING
A brush scratches the anti-reflective coating on a modern panel's glass. This coating is a few hundred nanometres thick, usually porous silicon dioxide fired into the glass surface. It increases light transmission by approximately 2–3%. It is not a film. It cannot be replaced and it cannot be repaired.
A brush — especially one sliding over a layer of dust already deposited on the surface — leaves micro-scratches in this coating. The damage is twofold: the coating loses its optical effectiveness, and the scratches retain dirt, so the panel will become dirty more quickly from then on.
The anti-reflective coating cannot be restored. A single brush wash degrades it permanently.
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.