The Biggest Myth About Solar Attic Fans: “They Stop Working on Cloudy Days”

I still see this claim every week on LinkedIn:

“Solar attic fans are useless when it’s cloudy or raining.”

Reality: They work far better than most people think — even under thick clouds.

Here’s the truth, backed by physics and real-world measurements.

1. Modern solar panels don’t need bright sunshine to produce power

Today’s high-quality monocrystalline panels convert 20–25% of incoming light into electricity. A typical 40–60 W solar attic fan only needs about 6–8 W to run at full speed.On a heavily overcast day, sunlight intensity often drops to 100–300 W/m² (vs. 1000 W/m² on a clear day). That still delivers 4–15 W — more than enough to keep the fan spinning, just at a slightly lower speed.

2. What actually happens in cloudy weather

From a year-long monitoring project in Seattle (one of the cloudiest cities in the U.S.):

• On clear summer days → fan runs 12–14 hours at full power

• On partly cloudy days → 9–11 hours at 70–80% speed

• On typical overcast days → 5–8 hours at 40–60% speed

• On the darkest winter days → still 2–4 hours at low speed

Even on the gloomiest days, the system was still removing thousands of cubic feet of hot, moist air from the attic. Zero ventilation would have been far worse.

3. The hidden advantage nobody talks about

Ventilation effectiveness depends heavily on temperature difference, not just fan speed. On a cloudy 28 °C day, an unventilated attic can still climb to 50+ °C. Even a slow-moving solar fan exploits that massive temperature delta and keeps extracting heat when a traditional electric fan (waiting for its thermostat) would still be off.

4. For consistently cloudy climates

If you live somewhere with weeks of grey skies (think UK, Pacific Northwest, Northern Europe), look for models with a small built-in battery backup or ultra-low-voltage grid tie-in. They run 100% free on solar most of the year and automatically switch to a trickle of grid power only when needed — usually less than 5–10 kWh annually.

Takeaway

No, solar attic fans do not “stop working” when clouds roll in. They simply slow down a bit — and a slower fan is still infinitely better than no fan at all. In almost every climate, a well-designed solar attic fan delivers 80–90% of its rated yearly performance, clouds included.

So next time someone repeats the “it only works when it’s sunny” myth, feel free to share this post.Have you used a solar attic fan in a cloudy or rainy region? How did it actually perform for you?