The forecast says overcast all week, and you’ve got panels waiting on the roof or a portable set in the trunk. Are they useless? Not at all. Solar panels do work on cloudy days, though not nearly as well as in full sun. Here’s what to expect and what you can do about it.
Do solar panels work on cloudy days? Yes. Panels still make electricity from the diffuse light that gets through clouds, but output usually falls well below sunny-day levels. Heavy overcast can cut output dramatically, while thin clouds cost much less.
How do solar panels work on a cloudy day?
Sunlight reaches the ground in two ways. Direct light comes straight from the sun, and diffuse light scatters off clouds and the sky. Panels can use both, which is why they keep trickling out power under cloud cover.
The catch is that diffuse light is much weaker. Think of it as the difference between a bright bulb and a dim night light. You’ll still see something, just far less.
What output can you expect on cloudy days?
There’s no single number, since it depends on cloud thickness, time of day, panel type and location. As a rough guide, light or thin clouds may cost you a modest share of your output, while thick, dark overcast can leave you with only a small fraction of rated power.
- Bright, thin cloud: often still decent output.
- Solid gray overcast: a big drop, sometimes to a small percentage of rated watts.
- Storms and heavy rain: very low output.
Keep in mind that the rated watts on a panel come from ideal lab conditions. Even on clear days, you’ll rarely see the full number.
Shade is a bigger problem than clouds
Here’s something many people miss. A small shadow from a branch, a vent or a nearby building can hurt output more than a cloudy sky does, since it can block part of the panel while the rest keeps working. In a series string, one shaded panel can pull down its neighbors, too.
If you’re curious how wiring plays into this, see our explainer on series vs parallel solar panel wiring. Moving a panel a few feet into clear light can matter more than waiting for the clouds to move on.
What about winter?
Winter brings shorter days, a lower sun and often more cloud, so you get fewer hours of weaker light. Cold itself actually helps panel efficiency a little, but it doesn’t make up for the lost daylight. Snow can cover panels completely, though it often slides off tilted ones as the sun warms the glass.
For a winter system, plan for lower daily energy. A larger panel array, a bigger battery or a backup source can make up the difference.
How to get better results from portable setups
- Aim the panel at the sun. Even under cloud, pointing toward the brightest part of the sky helps.
- Adjust through the day. Re-aim every hour or two if you can.
- Get out of the shade. Place panels in open light, away from trees and walls.
- Keep panels clean. Dust, leaves and snow cut output.
- Bring extra capacity. A larger panel or a second one makes up for weak light.
- Use a good controller. MPPT often squeezes out more in dim light.
Looking for gear that handles variable light? Our roundup of the best portable solar panels for power stations and the guide to the best foldable 200 watt solar panels are worth a look. Or consider a complete unit from our list of the best solar generators.
If you want to charge a power station on weak sun days, also read how to charge a power station with solar panels.
It also helps to keep expectations in check. If you depend on solar for something important, like a fridge or a medical device, don’t count on a cloudy week. A spare battery, a car charger or a small generator can cover the gap. Treat clouds as a slowdown, not a shutdown, and plan your power use so the essentials get charged first on the sunny days you do get.
Frequently asked questions
How much power do solar panels make on a cloudy day?
It varies widely, from a fair share of rated output under thin clouds to just a small fraction under thick overcast. Expect far less than on a clear day.
Do solar panels work in the rain?
They do, at reduced output. Rain can also wash dust off the surface, which helps once the sun returns.
Can a portable power station charge in the shade?
Slowly, if at all. Panels need light on their surface, so move them into open sun or at least bright open sky.
The bottom line
Solar panels on cloudy days still work, just at a lower level. Avoid shade, aim your panels, and plan for extra capacity, and you’ll still collect useful power when the sky turns gray.