Understanding your humidity impact
Why humid air cuts output even under clear skies
A humid subtropical climate with 65% average annual humidity loses around 2.3% of output to atmospheric moisture. An 11,000 kWh/year system loses roughly 253 kWh a year under those conditions. Water vapor absorbs and scatters a portion of incoming sunlight before it reaches the panel surface, even on days with no visible clouds. This is a separate effect from cloud cover, which blocks light directly rather than filtering it through the atmosphere. A DOE assessment of federal solar systems found an average performance ratio of 78.6%, a benchmark against which humidity-driven losses can be measured.
Why climate type matters as much as the humidity number
Tropical and humid subtropical climates carry a deeper, more persistent layer of atmospheric moisture than arid or temperate regions, which scatters more sunlight throughout the year. Arid climates see very little of this effect even at moderate humidity readings, since the total moisture in the air column stays lower. The effect is strongest in hot, historically sun-rich regions, where higher temperatures raise both moisture and haze. NASA's POWER data viewer publishes the solar irradiance and weather records behind that pattern for any location, so the local numbers matter more than any national average.
The secondary heat-retention effect
High humidity also affects panel temperature indirectly. Humid air holds heat overnight and slows how quickly panels cool down between sunny periods, which compounds slightly with the direct light-scattering effect in very humid climates. This adds a modest extra loss on top of the direct scattering effect in the most humid regions.
How this compares to other climate factors
Humidity loss is typically smaller than shading, snow, or soiling loss, but it adds up over a full year in consistently humid climates. Pair this estimate with the dust and soiling loss estimator if your area also sees high pollen or particulate levels, since humid climates often see both effects together.
What to do with this number next
Run the real world vs nameplate output gap calculator to see how humidity loss fits into your system's overall performance picture. Once your output estimate accounts for humidity and other real-world losses, the payback period calculator can translate that adjusted figure into a more realistic timeline for when the system pays for itself.