The short answer
On lightly dusted panels the difference is small — a few per cent, and the first decent rain does much of that by itself. Where it does add up is a caked bottom edge, moss growth or bird droppings: those shade part of the cells, and because cells are wired in series that drags the whole panel down. That is the case in which cleaning pays for itself.
Panels do not get dirty evenly, and that is the heart of the whole story. Rainwater runs down the panel, picks up dust, and stops at the bottom edge against the aluminium frame. There it dries. Repeat that a few hundred times and a dark band of one to three centimetres forms across the full width.
That band is disproportionately expensive. Solar cells in a panel are wired in series, like the bulbs in old Christmas lights: the weakest decides what gets through. A row of cells that is a third in shadow throttles the whole string. Which is why a panel that looks perfectly fine from below can still deliver noticeably less than its neighbour on the same roof.
You can check this yourself, and it is the most honest way to decide whether cleaning is worth it. Almost every inverter has an app showing daily yield. Compare a clear day before the clean with an equally clear day after, in the same season. If you see no difference, your panels were clean enough — and next year they will not need doing either.
Be sceptical of anyone promising a fixed percentage. "Up to 30% more yield" is a figure from desert research where panels go months without rain. In the Netherlands it rains every few days on average, which is exactly why the honest numbers here are far lower.
Practically: once a year is enough for most roofs, ideally in early spring before the pollen arrives and with the summer yield still ahead of you. If your house sits under trees, on a country road or beside a field, twice is realistic. And do not walk on them yourself — a panel is not built to stand on, and a tiled roof is certainly not built to be walked across.
What to remember
- Dust costs little; a caked bottom edge, moss or bird droppings cost a lot.
- Check it yourself in your inverter app — it is the only honest evidence.
- Once a year in early spring is enough for most roofs.