Can my Solar Panels Generate Power During Cloudy Days and in the Winter?

Our clients often ask us if their solar panels will still generate power during cloudy days and by how much their system performance will change between summer and winter.

Solar panel performance in Portugal varies significantly between winter and summer and between sunny and cloudy days due to changes in solar irradiance (sunlight energy), temperature, and daylight hours. Here's a breakdown:

Summer vs Winter Performance

Summer (June to August)

• Up to ~15 hours of sunlight per day.
• Peak performance, especially in regions like the Alentejo and Algarve.
• Systems often generate 3 to 5 times more electricity/day than in winter.
• Panels can become slightly less efficient when very hot, but high irradiance compensates.

Winter (December to February)

• As little as ~9.5 hours of light.
• The sun sits lower in the sky, reducing intensity.
• Particularly in the north (e.g. Porto, Braga), leads to decreased output.
• Expect a 60–80% drop in energy production compared to summer.

Sunny vs Cloudy Days

Sunny Days

• Panels operate near their maximum capacity.
• Clear skies allow direct sunlight which is ideal for solar generation.

Cloudy Days

• Panels still generate power but at 10–30% of max output, depending on cloud thickness.
• Thin clouds might reduce output by only 20%, while thick overcast days can reduce it by 80% or more.

Example Output (6 kW system in Lisbon)

Summary

Portugal is ideal for solar: High annual irradiance (~1600 to 2000 kWh/m²/year).

Summer is your sweet spot: Most energy is generated from May to September.

Storage helps: Batteries can balance out low winter production.

Panel tilt & orientation: Optimising for winter sun (lower angle) can improve seasonal performance.