On a steep British roof, rain does most of the cleaning and a paid clean often recovers less generation than it costs. On a shallow or flat array, in a dusty, agricultural, coastal or polluted location, or after birds have been at it, the arithmetic reverses. The honest way to settle it is to measure, not to accept a claim.
Do solar panels need cleaning, or is rain enough?
Panels on a pitched roof shed water and the dirt with it, and a steady British autumn does the job reasonably well. Uniform light soiling costs a small percentage of output and is largely self correcting. What rain does not clear is bird fouling, sticky pollen and resin, agricultural dust, salt in exposed coastal positions, industrial deposition, and anything on a shallow pitch or a flat array where water sits rather than running. Lichen and moss creeping up from the roof edge is the other case that needs a hand.
How to tell whether it is worth it
Look at the monitoring. Compare this year's generation for the same months against previous years, and look at whether the decline is gradual and uniform, which suggests soiling, or sudden, which suggests a fault. If a cleaner claims a percentage gain, ask for the generation figures for the week before and the week after, on comparable weather. It is a measurable claim, so measure it.
Washing solar panels: methods that damage them
Abrasive pads and scrapers scratch the glass and the scratches stay. Hard water dried in place leaves mineral deposits that shade. High pressure jets can force water past seals and into junction boxes, and aiming one at a panel edge or an inverter is a way to turn a cleaning bill into a repair bill. Standing on panels cracks cells invisibly. The safe method is soft brush, pure deionised water on a pole from the ground, no detergent unless the manufacturer permits it, and nobody walking on the array.
Solar panel cleaning prices UK: access is most of the cost
Cleaning a ground mounted array or a single storey extension roof is cheap. Cleaning a three storey array that needs a tower or scaffold is not, and the access cost has nothing to do with how dirty the panels are. If access equipment is going up for another reason, that is the moment to have the cleaning and the bird proofing done as well, because the expensive part is already paid for.
Questions people ask about cleaning solar panels
How often should solar panels be cleaned?
On a steep, clean domestic roof, rarely, because rain does the work. On shallow or flat arrays, and in dusty, agricultural, coastal or polluted locations, an annual or biennial clean is more defensible. Let the generation figures decide rather than a schedule.
Does cleaning increase output?
It recovers output that soiling has taken, which is a real but usually modest amount on a well washed roof and a larger amount on a shallow, soiled or fouled one. Ask for before and after generation figures on comparable weather.
Can I clean solar panels on roof slopes myself?
From the ground with a soft brush and a water fed pole, reasonably. From a ladder or on the roof, it is work at height with a real fall risk, and walking on panels damages cells. If it cannot be reached safely from the ground, it is a job for somebody equipped for it.
What should not be used on solar panels?
Abrasive pads, scrapers, high pressure jets aimed at edges or the inverter, hard water left to dry, and detergents the manufacturer has not approved. Any of them can cost more than the soiling did.