Solar under cloudy skies — busting the “no-sun, no-power” myth

“Cloudy weather kills solar output.” We hear it all the time, yet most mid-latitude roofs still produce 60 – 80 % of their clear-sky energy on dull days. Modern mono-PERC and TOPCon panels even prefer cooler conditions. Let’s unpack what really happens when the clouds roll in—and why our Solar PV Sizer wizard already accounts for it.

1 · Diffuse irradiance keeps the amps flowing

Light scatters through cloud droplets, reaching the panel from every angle. Field data shows typical cloudy-day power drops of 20 – 60 % depending on cloud thickness and module quality.

Even in thick cloud, panels rarely fall below 40 % of clear-sky output.

2 · Cooler modules, happier electrons

High cell temperature knocks 0.34 % of voltage off for every °C above 25. Cloud cover lowers panel temps by up to 15 °C, partly offsetting irradiance losses. On cool, hazy afternoons you can sometimes see higher net yield than on hot, cloud-free noons.

3 · Mono, poly or thin-film — who wins in low light?

Monocrystalline PERC / TOPCon: retain > 80 % of rated efficiency at 200 W/m².

Polycrystalline: around 75 %.

Thin-film (CdTe, CIGS): roughly 70 %, but they wake up earlier and stay active later each day.

4 · What it means for system sizing

When you run the wizard, the worst-month model already folds seasonal cloudiness into the yield numbers via PVGIS data. You don’t need to oversize panels just for grey days—stick to the suggested kWp and let annual averages smooth things out.

5 · Key take-aways

Panels still generate under cloud; expect 40 – 80 % of sunny-day power.

Cooler module temperatures partially offset lower irradiance.

Choose high-efficiency mono-PERC if you live in persistently grey climates.

Judge payback on annual kWh, not a single sunny-day snapshot.

Ready to size a system that works rain or shine? Head back to the Solar PV Sizer wizard and watch the numbers update in real time.