Yes, more than dust does. A dropping blocks light from part of a cell completely, and because cells are wired in strings, a small shaded patch can pull down more than its own share of output. Rain rarely washes droppings off. A gentle clean with soft brushes and pure water removes them.
Why are droppings worse than dust?
Dust spreads thinly and lets most light through. A dropping is a solid, opaque patch that blocks light from the cell beneath it completely. Rain doesn't dissolve it well, so it tends to sit there, bake on in the sun, and get harder to shift.
Droppings are also slightly acidic, and left for a long time they can mark the glass and the frame. That's another reason not to leave them.
How can a small patch affect a whole panel?
Because of how the cells are wired. The cells in a panel are connected in series, in strings. When one cell is shaded, it limits the current that can flow through the rest of its string. Panels have bypass diodes to work around shaded sections, but the shaded section still contributes little or nothing.
The exact effect depends on your system: whether you have a single inverter, optimisers or micro-inverters, and where the dropping is. The point is that a small patch can cost more than its size suggests.
Will rain wash them off?
Usually not. Fresh droppings may soften in heavy rain, but most stay put, especially on shallow roofs. That's the main difference between droppings and dust: one cleans itself, the other doesn't.
Why do some roofs get so many?
Birds like a perch with a view. Ridge tiles, TV aerials, satellite dishes and nearby trees above the panels all give birds somewhere to sit, and the panels below catch what falls. Coastal homes in Essex get gulls; inland homes often get pigeons and crows.
If pigeons are nesting under the panels, that's a separate problem. Bird proofing, with mesh around the edge of the array, is a specialist job for a proofing company. We can clean the panels once it's done.
How do you get droppings off safely?
Soften them first, then brush gently. Wet the dropping with plenty of clean water, give it a few minutes, then use a soft brush to lift it. Rinse well. Don't scrape it off with anything hard, and don't use a pressure washer or abrasive pad; most panel makers warn against both.
Purified water is ideal, because it leaves no spots and no residue that could attract more dirt.
Can I do it myself?
Only if you can reach the panels safely from the ground, such as on a bungalow or a garden array. Never stand on the roof or rest a ladder on the panels. On most two-storey homes, the safe way is a long water-fed pole from the ground.
How do I know if droppings are costing me power?
Look at your generation figures on bright days. If one string or one panel is lower than the others and you can see droppings on it, cleaning is likely to help. If output has dropped and the panels look clean, speak to your installer, because that points to a fault rather than dirt.
Our solar panel cleaning removes droppings, lichen and film with pure water and soft brushes, from the ground, across Essex. For more on timing, read how often solar panels should be cleaned.