Soiling is the quiet thief of solar yield. Dust, soot, bird droppings and pollen settle on module glass and block a share of the light that should reach the cells. Around Dhaka and Gazipur, where brick kilns, construction sites and heavy traffic sit close to industrial roofs, the effect builds quickly during the dry season.
How quickly output drops
On the plants we monitor, the pattern is consistent. Losses are small in the first week after cleaning, then climb steadily from November to March when there is no rain to wash the glass. Arrays within a few kilometres of kilns or highways lose output faster than those in cleaner surroundings, and low-tilt roofs lose more than steeper ones because dust is not shed by gravity.
Indicative soiling loss by weeks since cleaning, dry season
Illustrative profile for a low-tilt industrial roof near Gazipur.
What it costs
On a 1 MWp plant generating around 1,300 MWh a year, every percentage point of average soiling loss is roughly 13 MWh of lost energy. At a blended tariff of about Tk 12 per unit, that is in the region of Tk 1.5 lakh a year per point. A plant averaging 5% loss across the dry months can be losing several lakh taka a year to dust alone.
Not all dirt is dust
Uniform dust is the easiest soiling to deal with, because it lowers output evenly and washes off. Other deposits cause more damage than their size suggests:
- Bird droppings create hard shading on individual cells, which can force bypass diodes to operate and create hot spots.
- Soot and oily residue from kilns, generators and exhaust vents bonds to glass and needs more than water to remove.
- Dirt lines along the lower frame build up on low-tilt modules where rainwater pools, shading the bottom row of cells.
- Leaves and debris in gutters and between rows trap moisture and dust against the modules.
Thermal imaging during routine maintenance shows whether localised soiling is creating hot spots that could shorten module life, not just reduce this month's output.
Setting a cleaning frequency
There is no single right answer, because the cost of cleaning has to be balanced against the value of recovered energy. As a starting point we recommend:
- Every two weeks in the dry season for arrays near kilns, highways or construction.
- Monthly in the dry season for arrays in cleaner surroundings.
- After storms and dust events regardless of schedule.
- Reduced frequency during the monsoon, when rain does much of the work.
Let the data decide
The best cleaning schedule is the one your monitoring data justifies. Comparing performance ratio before and after each clean shows exactly how much energy is being recovered, and whether an extra visit pays for itself. That analysis is part of every preventive maintenance contract we run.
Written by
O&M Desk
Monitoring & maintenance team, Emerged Power
Field notes from the team that keeps our plants clean, cool and online.