Understanding your cloud cover loss
Why cloudy days still produce meaningful power
A location with 50% average annual cloud cover and mixed cloud density loses around 7% of annual output. An 11,000 kWh/year system loses roughly 770 kWh a year under those conditions. Panels still generate meaningful power from diffuse light that passes through clouds, typically 10% to 50% of clear-sky output depending on cloud thickness. A fully overcast day is not a zero-production day. NASA's POWER solar data project tracks this same diffuse-versus-direct irradiance split from satellite observations, which is part of why cloud-adjusted output estimates can vary so much by exact location even at similar average cloud cover.
Cloud density changes the math more than cover percentage
Thin, broken clouds let a large share of sunlight through, while thick overcast layers block far more light per hour of coverage. Two locations with identical cloud cover percentages can see very different output losses based on typical cloud type. A marine layer or persistent low overcast common along parts of the West Coast behaves very differently than the scattered afternoon clouds typical of many inland climates. That variability is also why modeled output and measured output can diverge: according to a DOE report on federal solar performance, weather-driven variability is one of the largest sources of that gap.
How system design can recover some lost output
Systems with better diffuse light capture, including some bifacial panels and certain inverter configurations, recover a modest amount of the output that would otherwise be lost under cloud cover. This isn't a dramatic difference, but it can matter in consistently cloudy climates where the gain compounds across many overcast days per year. Microinverters and power optimizers tend to hold up better than a single string inverter on a patchy, partly-cloudy day, since a shadow crossing one panel doesn't drag the whole string down with it, a distinction the microinverter vs string inverter output tool breaks down in more detail.
How this compares to other atmospheric factors
Cloud cover loss is a direct light-blocking effect, distinct from the atmospheric scattering covered in the humidity impact calculator. Coastal properties often deal with both cloud cover and the separate effect covered in the coastal fog impact calculator.
What to do with this number next
Run the real world vs nameplate output gap calculator to see how cloud cover loss fits into your system's overall performance picture alongside other factors.