5 min read · Updated 1 August 2026
How much output do dirty solar panels really lose?
Soiling typically costs a South African solar system between 5% and 25% of its output, and in extreme cases more — one Midstream client measured 32% higher generation after a professional clean.
The range, and why it is so wide
Light dust in a suburb with regular rain might cost 5%. A dry-season Highveld array with pollen and bird droppings commonly sits in the 15% to 25% band. Farms, mines and construction-adjacent sites can go higher still. One Midstream client measured 32% higher generation after our team cleaned their system.
The range is wide because soiling is not uniform. Two houses on the same street can differ by a factor of three depending on tree cover, roof pitch, prevailing wind and how close they are to a gravel road.
Why a little dirt causes a big loss
Panels are wired in series strings. Within a panel, cells are grouped into sub-strings protected by bypass diodes. Current through a string is limited by its weakest cell, so a band of dirt across the bottom row of cells does not cost you the area of that band — it can pull down the whole panel, and in some layouts drag on the string.
This is why the streaky lower edge left by light rain matters more than an even light haze across the whole array, and why partial cleaning is not worth doing.
Putting it in rands
Take an 8 kWp residential system generating roughly 1,150 kWh a month in a good Gauteng month. At around R2.80 per kWh of avoided grid purchase, that is about R3,200 of value. A 15% soiling loss is roughly R480 a month, or R5,700 a year.
Cleaning 20 panels at R35 each is R700 excluding VAT. On those numbers the clean pays for itself in well under two months, and everything after that is recovered generation. Commercial sites at R25 per panel with far larger arrays see the payback even faster.
How to measure it on your own system
Pick a clear, cloudless day and note peak output in kW around midday from your inverter app. Have the array cleaned, then take the same reading on the next clear day. Compare like with like: same time of day, same season, no new shading.
If you have per-string or per-optimiser monitoring, compare strings before the clean. A single underperforming string with no shading explanation is very often soiling or a dirt-trapping obstruction rather than a hardware fault.
Beyond output: what soiling does over time
Bird droppings are acidic and can etch glass and create permanent hot spots where a cell is repeatedly shaded and forced to dissipate heat. Persistent hot spots degrade cells, discolour the backsheet and shorten panel life.
Organic build-up along the bottom frame also holds moisture against the seal, which accelerates delamination. Regular cleaning is as much about protecting a large capital asset as it is about this month's bill.
Common questions
- Will cleaning restore my system to 100% of its original output?
- It restores the losses caused by soiling. Panels also degrade naturally by roughly 0.5% a year, and shading, faults or an ageing inverter cause losses that cleaning cannot recover. If output stays low after a clean, that points to a fault worth investigating.
- How quickly do panels get dirty again?
- In Gauteng's dry season, measurable soiling can return within 8 to 12 weeks. In the rainy months it is slower. That is why most homes settle on two cleans a year and commercial sites on four.
Want your panels checked?
RobCon Electrical cleans residential systems from R35 per panel and commercial from R25, across Pretoria, Centurion, Midrand, Johannesburg and greater Gauteng.