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 3–5% 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