Clear answer, explained.
Panel soiling — accumulation of dust, pollen, bird droppings, and debris on panel surfaces — reduces light reaching the solar cells and lowers output. In Canada's climate, soiling is generally less severe than in arid regions because spring and fall precipitation naturally cleans panels on tilted rooftops. However, sustained dry periods in summer can allow soiling to accumulate enough to reduce system output by 2–5% or more.
Bird droppings are the most impactful soiling type because they create concentrated shading on specific cells. A single dropping on part of a cell can disproportionately reduce output from the entire string connected to that module due to how bypass diodes manage partial shading. For facilities where birds congregate — particularly near HVAC equipment, drainage features, or adjacent trees — soiling from bird droppings can be a recurring issue that warrants preventive measures such as perimeter mesh under panel arrays.
For most Ontario commercial systems on tilted rooftops, natural rainfall provides sufficient cleaning throughout the year without professional intervention. Flat or very low-tilt systems — common on warehouse and industrial roofs — do not benefit from the same gravity-assisted cleaning and may benefit from annual professional cleaning if located in areas with significant particulate accumulation during dry periods. Soiling impact is most pronounced during July and August when production potential is highest.
What this means in practice.
- Soiling typically reduces output 2–5% during dry summer periods
- Bird droppings create concentrated shading affecting entire strings
- Ontario climate: spring and fall rainfall cleans most tilted systems naturally
- Flat and low-tilt systems benefit more from scheduled cleaning
- Summer soiling is most costly given peak production potential
- Perimeter mesh prevents bird nesting and reduces droppings accumulation
Best-fit environments.
- Systems on flat or very low-tilt rooftops in dry summer periods
- Facilities near bird populations with recurring soiling issues
- Systems in areas with significant particulate or industrial fallout
- Installations where summer production has been lower than modelled
