Why dirt can reduce solar production
Surface material reduces the light reaching the cells. The National Laboratory of the Rockies lists soiling as one category of system loss in PVWatts, separate from shading, age, wiring and other losses. A model default describes an assumption, not a measurement of your roof.
Concentrated obstructions and a light film do not necessarily behave the same way. The result depends on the module design, bypass diodes, electrical arrangement and the obstruction. It is inaccurate to promise that one bird dropping always reduces the output of the entire array by a set amount.
Identify what cleaning is expected to change
Describe the visible condition before comparing prices. A service aimed at loose dust is different from under-panel nesting cleanup or a mineral-deposit assessment.
Scroll sideways to see all columns.
| Observation | What it might involve | What to ask |
|---|---|---|
| A light even dust film | Accessible surface soiling | Is cleaning useful now, or should conditions be reviewed later? |
| Bird droppings or a leaf pile | A concentrated obstruction | Can it be safely removed, and is there a source that needs attention? |
| Persistent white spotting | Possible mineral deposits or damage | Is a test area or separate treatment needed? |
| Birds entering below modules | Under-panel activity beyond glass washing | Does the quote include inspection, cleanup and exclusion? |
| A new shadow across the array | Tree or structure shading | Who can assess or address the shade? Washing will not remove it. |
| A sudden unexplained production drop | Several possible system or site causes | What do the alerts and system provider show before paying for a clean? |
A before-and-after comparison needs more than two sunny days
Use daily energy readings in kilowatt-hours across nearby dates, and record the cleaning date. Retain several days on each side of the visit so an unusual day does not decide the result. Check for missing monitoring data, outages, changing shade or a different system configuration.
Similar-looking skies are not identical operating conditions. Sunlight intensity, module temperature and inverter limits can change production even on clear days. A household comparison is a useful observation; a measured soiling-loss claim needs suitable weather data or a controlled clean-versus-soiled reference. Do not attribute the entire difference to cleaning.
Keep the calculation separate from the conclusion
For two comparable daily readings, the observed change is (after minus before) divided by before, multiplied by 100. For example, 30 kWh before and 31 kWh after is about 3.3% higher production. Those illustrative numbers are not a CA Solar Wash result or forecast.
The arithmetic alone does not say why production changed. Label the result as an observed difference and record the comparison conditions. If you need a contractual performance test or a warranty finding, arrange that through the appropriate system provider rather than an exterior cleaning visit.
Decide whether the visit is worthwhile for your property
Ask what material will be removed, whether an underlying issue is addressed and what the visit costs. A useful comparison includes repeat buildup, access, bird cleanup and any maintenance obligations, rather than assuming all value comes from a single electricity reading.
Do not multiply one unusually good day into a guaranteed annual saving. The value of additional electricity depends on how it is used and your actual utility arrangement, while soiling and weather change over time. No payback period is promised here. If the glass is lightly soiled and there is no other maintenance need, waiting may be reasonable.
Know when cleaning is the wrong first booking
A sudden drop, inverter alert, cracked glass or visible connection damage needs the system provider to assess it. A rain event is not a diagnostic test: rain may leave deposits behind, and production can change for unrelated reasons. Persistent low output does not automatically mean dirt or an electrical fault.
CA Solar Wash can provide accessible cleaning and photographs of visible conditions. Ask for the planned method to be checked against the module instructions. Neither clean-looking glass nor a photograph certifies electrical performance or guarantees manufacturer warranty coverage.
