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How much generation do I lose if my panels are dirty?

Short answer

In a rainy climate like the UK, typical dirt costs a few percent a year, not tens of percent. A European study found average soiling losses of 0.9% to 5.3% a year depending on how well rain cleans panels, and Loughborough University reports UK losses above 5% in some cases. Bird droppings and moss cause bigger, localised losses.

Last checked 24 September 2026 · 2 sources

In more detail

  • Peer-reviewed European mapping study: mean annual soiling loss 0.9% if rain cleans perfectly, 5.3% if rain is only 10% effective; losses vary year to year.
  • Loughborough University (2021 press release): soiling can cut output by more than 5% in the UK.
  • Localised soiling (droppings, leaves, lichen along the lower frame) matters more than even dust because a shaded cell can force a bypass diode to switch off a third of a panel.
  • Economics: on a 4 kWp system making ~3,500 kWh/yr, a 3% loss is about 105 kWh/yr; compare with the cost of a professional clean (EST: GBP 20-200).

The facts, dated

  1. 01
    European mean annual PV soiling loss, rain as perfect cleaner — 0.9 %/yr

    market_data · n/a · checked 24 September 2026 · elib.dlr.de

  2. 02
    European mean annual PV soiling loss if rain cleaning effectivity is 10% — 5.3 %/yr

    market_data · n/a · checked 24 September 2026 · elib.dlr.de

  3. 03
    Soiling can reduce UK PV output by more than 5% (Loughborough University) — >5 %

    market_data · UK · checked 24 September 2026 · lboro.ac.uk

What we saw in the field

[QA review 2026-09-24] I-1 CONTRADICTS as a general rule (soiling 0.9-5.3%/yr). | Earlier notes: Consistent with installers recommending cleaning where access is easy, but evidence suggests annual gains of a few percent: cleaning a high roof every six months is unlikely to pay back unless droppings or low tilt are involved.

Sources

Last checked: 24 September 2026