Do solar panels work on cloudy days and in winter?
General information, not a substitute for the advice of an attorney.
Short answer: Yes. Panels generate whenever light reaches them, but less when the sky is overcast or the sun is low. The Department of Energy puts it plainly: "One challenge facing solar energy is reduced energy production when the sun sets or is blocked by clouds" (DOE Solar Energy Technologies Office). How big the winter dip is depends on where you are, and the right way to find out is to run your own site through NREL's PVWatts rather than trust a rule of thumb.
Why output drops
- PV cells "produce the most electricity when they are directly facing the sun," so a low winter sun angle on a fixed array costs output; that is also why fixed panels are usually angled south in the Northern Hemisphere (EIA).
- Clouds block part of the light. DOE lists cloud cover alongside nightfall as the two causes of reduced production (DOE).
- Snow and dirt on the glass are a separate loss. PVWatts has a monthly irradiance-loss input that "can be used to represent soiling or snow losses" (NREL PVWatts).
Seeing the seasonal pattern for your address
PVWatts uses typical-meteorological-year weather from NREL's National Solar Radiation Database (PSM V3 TMY 2020 in version 8), which includes each location's typical cloud cover (NREL PVWatts). Its outputs include ac_monthly, the AC kWh for each month January through December, and solrad_monthly, the solar radiation in kWh per square meter per day for each month (PVWatts V8 documentation). Comparing December to June in that table tells you your own winter-to-summer ratio. We do not publish a single national percentage for the winter drop because it varies by latitude, climate, and tilt, and no primary source we opened states one.
What it means for sizing
Because annual output is what a bill or a lease pays on, the useful number is PVWatts' annual kWh and capacity factor rather than any one month. For context, U.S. utility-scale solar PV averaged a 24.0% capacity factor over all of 2025, cloudy days and winter included (EIA Electric Power Annual, Table 4.08.B).
Related
See rooftop solar and net metering by state, how long solar panels last, and outage history by county and ZIP.
Related questions
General information from the sources linked above, not legal, tax, or financial advice.
Sources and helpful links
- DOE Solar Energy Technologies Office energy.gov
- EIA eia.gov
- NREL PVWatts pvwatts.nrel.gov
- PVWatts V8 documentation developer.nrel.gov
- EIA Electric Power Annual, Table 4.08.B eia.gov
More official contacts and resources: Helpful links.