How often to clean solar panels in South African conditions
Clean solar panels every 3 to 6 months in most of South Africa, every 4 to 8 weeks near farmland, gravel roads or the coast, and within a week of any dust storm. The interval is set by what settles on the glass locally, not by the calendar.
Cleaning interval by environment
Soiling rate varies more across South Africa than across most countries, because one national grid serves a winter-rainfall coastline, a summer-rainfall interior and a semi-desert. The table below sets the interval by dominant soiling agent rather than by province.
| Environment | Interval | Main soiling agent | Reason |
|---|---|---|---|
| Suburban, paved, coastal city | 4 to 6 months | Fine dust, salt haze | Rain removes loose dust; the salt film needs washing |
| Suburban inland, summer rainfall | 3 to 4 months | Dust, pollen | Dry winter months build a continuous layer |
| Within 500 m of a gravel road | 4 to 8 weeks | Road dust | Every vehicle pass deposits a fresh layer |
| Farmland, ploughed or harvested | 4 to 8 weeks | Soil, chaff, spray drift | Seasonal peaks at ploughing and harvest |
| Northern Cape, Karoo, arid interior | 6 to 10 weeks | Wind-blown dust | Little rain between cleans to interrupt build-up |
| Within 1 km of the sea | 6 to 8 weeks | Salt aerosol | Salt draws moisture and holds dust against the glass |
| Industrial or mining area | 4 to 6 weeks | Particulate fallout | Deposits bind chemically and resist rain |
What soiling costs between cleans
Soiling loss is the difference between the output of a clean reference panel and a soiled one. Research published by the United States National Renewable Energy Laboratory puts typical annual soiling losses in the low single digits at wet temperate sites, rising steeply at arid and dusty ones. The California field study by Kimber and colleagues in 2006 recorded losses accumulating at roughly 0.2 percent per day through a rain-free period.
At 0.2 percent per day, a 90-day dry winter on the Highveld ends with the array 18 percent down. That figure is the honest case for the intervals in the table above, and it is measurable on any inverter that logs daily yield.
Signs the array needs cleaning before the interval
- Daily yield falls against the same month last year with no shading change.
- A visible film shows when the glass is viewed at a low angle rather than from directly below.
- One string reads lower than its twin, which points at localised soiling or droppings.
- Water beads and sits on the glass instead of sheeting off.
- A dust storm, veld fire or harvest has passed within the week.
Rain does not replace a clean
Rain removes loose dust and leaves the bonded layer behind. Light rain performs worse than no rain at all: it wets the dust, draws it into a slurry, then dries it into a bonded film that resists the next rainfall. The pattern is familiar on any car left outside through a Highveld thunderstorm.
Heavy rain after a long dry spell recovers part of the loss. It recovers none of the loss from bird droppings, lichen or industrial fallout, all of which bond to the glass surface.
Droppings reset the interval
A single dropping shades a cell completely rather than dimming it. The bypass diode in that substring carries the current instead, so the loss is measured in whole substrings rather than in percentage points of light. An array with a bird colony under it needs cleaning on a bird schedule rather than a dust schedule, which is why solar panel bird proofing is treated separately.
Setting an interval from measurement rather than a table
The table above is a starting point. The accurate interval comes from the inverter log: clean the array, record daily yield for a clear week, then watch for the day yield falls 5 percent below that baseline under comparable weather. That number of days is the site interval, and it holds until the surroundings change. Method for the clean itself is in how to clean solar panels.
Questions
Do solar panels need cleaning in winter?
Yes, and in most of South Africa winter is when they need it most. The summer-rainfall interior gets almost no rain between May and August, so soiling accumulates without interruption through the driest months.
Does rain clean solar panels?
No, not completely. Rain removes loose dust and leaves bonded soiling behind, and light rain makes the film worse by binding dust to the glass as it dries.
How much output does a dirty solar panel lose?
Between 2 and 25 percent, set by location and time since the last clean. Field research records accumulation near 0.2 percent per day through rain-free periods, so the figure depends on how long the dry spell has run.