Understanding your coastal fog impact
What a typical coastal fog pattern costs
With moderate fog density clearing by 10:30 AM on 120 days a year, a system might see a net loss of around 6.9% of annual output, after accounting for the natural cleaning benefit fog condensation provides. This figure varies significantly by exact location, since coastal fog intensity can differ meaningfully even between nearby towns. A DOE assessment of 75 federal PV systems found meaningful year-to-year variation tied to local weather patterns, consistent with the kind of swings fog introduces.
Why burn-off timing is the dominant factor
Fog that clears by mid-morning affects far fewer productive hours than fog that lingers until midday, since the affected window runs from sunrise until the fog burns off. A site with a late-clearing marine layer loses a much bigger share of its morning production, which is why burn-off time matters more here than almost any other single input. See the morning vs afternoon shading tool for a related look at how morning-hour loss compares to afternoon-hour loss.
The natural cleaning benefit unique to foggy coastal climates
Yes, somewhat. The condensation that coastal fog leaves on panels naturally rinses away dust and light debris more frequently than in drier inland climates, which reduces the ordinary soiling loss that would otherwise accumulate between manual cleanings. This is a genuine partial offset worth factoring into the full picture, not just a footnote. That same persistent dampness is worth keeping in mind for equipment beyond the panels themselves, since damp, salt-laden coastal air is a known stressor for outdoor electronics, something the inverter lifespan estimator can help put a number on.
How fog differs from ordinary cloud cover
No. Coastal marine fog follows a distinct daily pattern tied to temperature differences between land and ocean, typically thickest at dawn and clearing predictably by late morning, while general cloud cover can occur at any time of day without that same reliable pattern. USGS research on Pacific coastal fog traces this same daily rhythm back to how the marine layer forms offshore over cold water and then advects onshore as inland areas heat up, which is exactly the mechanism behind that predictable morning-to-midday burn-off pattern. This predictability is actually useful for planning, since fog-affected hours are more consistent than random daytime cloud cover.
What to do with this number next
Compare this net figure against the dust and soiling loss estimator to see how much of the natural cleaning benefit would otherwise have been lost to soiling in a drier climate. The altitude impact calculator is also worth checking if your coastal site sits at a notable elevation. If you want to see what that loss means in dollars and years, the payback period calculator turns the same output figure into a timeline.