The short answer
Usually not for dust alone. NREL puts typical annual soiling losses at about 1–2% where rain is regular, including most of Europe.
Plain explanation
NREL's FY22 soiling briefing states annual soiling losses are low, 1–2%, in regions with regular rainfall such as the eastern USA or most of Europe. Spot-clean bird droppings and heavy pollen; leave ordinary dust to the rain unless monitoring shows a real drop.
Steps from the source
- Assume ~1–2%/yr soiling in rainy climates unless data says otherwise
- Spot-clean bird droppings and sticky pollen
- Use monitoring before selling a full wash contract
"Low, 1-2% in regions with regular rainfall such as the eastern USA or most of Europe"
Word for word from the source.
Source
NREL: PV Soiling Losses: Measurements, Modeling, and Mitigation Strategies (NREL/PR-5K00-83486), PV Soiling: Background — Energy losses vary — Low, 1-2% … most of Europewww.osti.gov ↗
Where installers ask this
- Cleaning solar panels in rainy climate? (r/solar)
- reddit.com/r/solar/comments/5xm1xj/wash_solar_panels_with_rainwater/
- reddit.com/r/DeutschePhotovoltaik/comments/1swzfmw/solaranlage_reinigt_sich_durch_den_regen_doch_nur/
General guidance based on the manufacturer's manual or the named source. Always follow the current manual for your exact model and your national code. DC and AC electrical work is for qualified installers only. Not affiliated with or endorsed by any brand named; trademarks belong to their owners.