Why Washing Solar Panels Matters: Benefits, Risks, and Best Practices

Solar PV systems are designed to operate reliably for decades, but like any piece of outdoor infrastructure, their performance is influenced by environmental conditions. One of the most common—and most overlooked—factors impacting solar production is soiling: the accumulation of dust, pollen, agricultural debris, bird droppings, and other contaminants on the surface of solar modules.

For owners of commercial-scale solar portfolios, especially in California, understanding when and how to wash solar panels can materially affect both energy production and financial outcomes.

Production Impacts from Soiling

Soiling reduces the amount of sunlight reaching the photovoltaic cells, directly lowering energy production. Numerous field studies and operating datasets (e.g., NREL, ScienceDirect) across show that soiling impacts can commonly drive losses of 15–30% or more. In California, these losses are exaggerated by the fact that these losses typically occur in the summer through fall:

•    When solar production should be at its peak
•    When utility prices are at their highest

In most regions throughout California, long dry seasons often mean that natural rainfall is insufficient to meaningfully clean modules, allowing soiling losses to compound over time.

Financial Implications for Commercial-Scale Solar Projects
Lost production translates directly into lost value—but the magnitude depends on market structure, ownership model, and tariff design.

For commercial-scale solar PV systems in the California electricity market, soiling losses can impact:

•    Reduced Utility Bill Savings under NEM or RES-BCT
•    Reduced Renewable Energy Certificate (REC) Revenues

These soiling losses can easily equate to hundreds of thousands of dollars in losses annually across multi-megawatt portfolios. The key is determining whether the incremental production recovered through washing exceeds the cost of the service—a question best answered with site-specific data.

Best Practices for Washing Solar Panels
Not all washing programs are created equal. Based on operating experience across commercial and public-sector portfolios, best practices include:

Wash Frequency
•    Most commercial sites benefit from one to two washes per year
•    High-soiling environments may justify targeted or seasonal washes

Timing
•    Schedule washes during peak production seasons (mid-to-late summer)
•    Avoid washing immediately before forecasted rain events
•    Use performance data to identify periods of elevated soiling losses

Methods
•    Use deionized water and soft-brush or low-pressure systems
•    Avoid chemicals or abrasive tools that can damage module coatings
•    Ensure vendors follow manufacturer and warranty-compliant procedures

Data-Driven Decisions
•    Pair washing with performance monitoring and trend analysis
•    Validate post-wash production gains to inform future maintenance planning

How Transform Energy Asset Management Can Help
Effective panel washing isn’t about doing more—it’s about doing the right amount, at the right time, for the right reasons.

Transform Energy Asset Management helps commercial and public-sector solar owners:
•    Quantify soiling losses performance data
•    Evaluate the financial return of panel washing 
•    Develop right-sized maintenance programs aligned with portfolio economics

If you’re unsure whether your solar assets being properly maintained, we’re happy to help assess your portfolio and design a data-driven maintenance strategy. Reach out to Transform Energy Asset Management to learn more.