By the PPA Funding team Last updated
Commercial solar panels lose output as dust, bird mess, moss and pollen build up and block light. On most UK roofs, one or two professional cleans a year keeps losses low. Cleaning uses purified water and soft brushes or poles, never harsh detergents or pressure washers that can damage the glass.
Why do commercial solar panels get dirty?
Panels sit at an angle in the open, so they collect whatever the local environment throws at them. Common culprits are dust and general grime, tree pollen in spring, bird droppings, and moss or lichen creeping in from the frame edges. Sites near farms, quarries, busy roads, food factories or the coast tend to soil faster because of airborne dust, salt or exhaust.
Rain helps, but it does not fully clean a panel. It tends to run off and leave a tide line of dirt along the bottom edge of each module, where soiling concentrates. Bird droppings are the worst offender because they are opaque and stick hard, shading individual cells rather than dimming the whole panel evenly.
How much output does soiling actually cost?
Soiling reduces output because anything on the glass blocks light reaching the cells. A light film of dust might cost a small single-digit percentage of generation; heavy or patchy soiling, especially bird mess, can cost noticeably more on the affected modules. Because a PPA charges you per kWh generated, cleaner panels mean more of the low-cost electricity you already pay a fixed rate for.
On a large roof, even a few percent across the year adds up, and the cost of a professional clean is usually small against the value of the output it recovers. Ask your installer or funder to model the likely gain for your site before booking work.
How often should a commercial array be cleaned, and how?
As a general guide, one or two cleans a year suit most UK commercial roofs, with dirtier sites needing more. Many operators tie cleaning to a monitoring alert: if generation drifts below the expected curve for the weather, it is often soiling. Booking a clean before the high-output spring and summer months protects the best generating period.
Cleaning should use deionised or purified water with soft brushes or fed poles, or a trained team with roof access equipment for bigger arrays. Avoid pressure washers, tap water that leaves mineral spots, and abrasive pads, all of which can scratch the anti-reflective coating. Never walk on panels. Most owners use a specialist rather than in-house staff for safety and warranty reasons.
This guide is general information, not financial or tax advice. Your circumstances determine what applies — please confirm with your accountant or advisor. Get a tailored proposal →
Frequently asked questions
Can I just let the rain clean my solar panels?
Rain removes some loose dust but leaves a dirt line along the lower edge and does nothing for bird droppings or moss. Most commercial arrays still benefit from one or two proper cleans a year.
Will cleaning damage the panels or void the warranty?
Not if done correctly with purified water and soft brushes. Pressure washers, harsh chemicals and walking on modules can cause damage and breach warranty terms, so most owners use a trained specialist. Check your manufacturer’s cleaning guidance.
Related guides
- Bird-Proofing Commercial Solar Panels: Pigeons, Gulls, Damage and Mesh Solutions
- Agrivoltaics: Dual-Use Solar on Farmland for Grazing, Crops and Farm Income
- Solar PV With Heat Pumps for Business: Heating and Hot Water From Your Own Power
See our funding options, commercial solar and solar by industry pages.