Winter rain streaks, when rain helps vs hurts.
Short answer: Steady winter rain partially cleans panels by rinsing loose dust. Sporadic light rain makes things worse, it lifts dirt into solution, which then evaporates leaving mineral streaks worse than the original dust. December is also bird-buildup peak: pigeons and starlings nest under panels during winter, and droppings accumulate on the glass below. The December check addresses bird issues, mineral streaks, and inspects the system before the low-sun months when production is hardest to recover.
The rain-cleaning myth
"My panels will rain-clean themselves" is partly true and mostly wrong. Here's the actual mechanism:
When rain helps
- Heavy steady rain (1"+ in 24 hours), sheets of water lift loose dust and carry it off. Single-pass cleaning effect.
- Multi-day rain events, sustained washing removes most loose particulates over 48+ hours.
- Rain following recent dust deposition, within 2 weeks of dust settling, before bake-on.
When rain hurts
- Light morning sprinkle, wets the dust into a slurry, then sun dries it into a streaked film harder to remove than original dust.
- Brief rain on hot panels, flash-evaporation leaves mineral deposits (calcium, magnesium from the water itself).
- Rain after long unwashed period, the baked-on summer film doesn't dissolve in rain; rain creates streaks instead of cleaning.
Mineral streaks
Davis tap water is moderately hard. When rain (which is soft) mixes with residual dust + minerals from any prior cleaning attempts using tap water, the evaporation leaves a streak pattern on the glass. These streaks reduce production 3 to 5% across affected panels.
Network specialists use softened/deionized water specifically to avoid this. The result: streak-free cleaning that doesn't add mineral residue.
Bird buildup in December
Pigeons and starlings shelter under solar panels during winter rain. By December, accumulated droppings can:
- Stain the panels below the gap (cosmetic but visible)
- Block panel drainage paths, causing water pooling
- Attract more birds (territorial signal)
- Generate ammonia compounds that can corrode mounting fasteners
The December check addresses this, including assessing whether bird-proofing should be added in spring.
The December inspection scope
- Clean panels (soft-brush + softened water)
- Remove bird buildup from under-panel gaps
- Inspect mounting hardware torque + corrosion
- Check connector dryness (winter rain can intrude on aged connectors)
- Production curve audit, low-sun months are when wear shows
- Recommend bird-proofing if bird activity is significant