Solar panel soiling and cleaning: what the research shows

01

Soiling can reduce solar energy production

Dust, pollen, pollution, bird droppings, and other debris can block sunlight from reaching solar cells and reduce a system’s energy production.

The U.S. Department of Energy and IEA PVPS describe soiling as accumulated material that blocks sunlight and can decrease photovoltaic energy output.

02

Average global losses are estimated at 4%–7%

IEA PVPS estimates that soiling is responsible for approximately 4%–7% of photovoltaic energy losses globally.

This is a global industry estimate, not a prediction for an individual Inland Empire property.

Important context: Individual results depend on location, weather, system design, panel angle, and the type of buildup present.

03

Southern California can experience larger temporary losses

NREL research indicates that peak Southern California soiling losses may exceed 15% under certain conditions, while typical annual losses are often 5% or less.

A temporary peak is not the same as an annual average and does not establish the loss at a particular home.

Important context: Actual losses vary with weather, system design, panel angle, surrounding activity, and buildup.

04

California sites showed gradual losses during dry periods

A California study measured average soiling losses of approximately 0.05% per day between rainfall or cleaning events, with substantial variation among sites.

The daily figure should not be multiplied into a guaranteed monthly or annual loss.

Important context: Accumulation depends on rain, wind, panel angle, weather, and site conditions.

05

Panel angle can influence accumulation

Low-angle solar panels may accumulate more soiling than more steeply tilted panels.

Flat and low-slope arrays can be more susceptible to accumulated dirt, but roof design, rainfall, wind, and local conditions also matter.

06

Rain does not always fully clean solar panels

Rain may rinse away some loose dust, but sticky pollen, bird droppings, and bonded deposits may remain.

NREL research found that rainfall was not sufficient to remove certain types of pollen from tested photovoltaic modules.

Important context: Results depend on the buildup, rain intensity, panel angle, and local conditions.

07

Cleaning needs are site-specific

There is no single cleaning schedule that is right for every solar system.

Frequency should reflect visible buildup, production data, weather, nearby vegetation or agriculture, bird activity, panel angle, and manufacturer guidance.

08

The financial value of cleaning varies

The value of cleaning depends on lost production, system size, electricity rates, and cleaning cost.

UC San Diego research found that cleaning typical residential systems was not always financially worthwhile when soiling was modest.

Important context: No fixed production increase or financial savings is promised.

09

Cleaning addresses removable soiling—not every performance problem

When removable dirt or debris is suppressing production, cleaning can recover some of the output lost to that buildup.

Cleaning does not correct shading, inverter faults, wiring issues, equipment degradation, temperature-related losses, or other causes of underperformance.

10

Cleaning should follow manufacturer guidance

Solar modules should be cleaned using non-abrasive methods consistent with the panel manufacturer’s care instructions.

Ask the service professional to explain their process and confirm that it is appropriate for the panel manufacturer and site conditions.