# Do solar panels work on cloudy days? What the data shows

URL: https://solarrealcost.com/does-solar-pay/do-solar-panels-work-on-cloudy-days
Updated: 2026-10-26
Review: Reviewed by SolarRealCost Editorial Team, editorial review: primary sources checked and independently fact-checked

Do solar panels work on cloudy days? Yes, at lower output. National-lab data for Seattle, Pittsburgh and Phoenix shows what clouds and winter cost a year.

Do solar panels work on cloudy days? Yes, at lower output. Thick cloud can block all of the direct sunlight, but panels still get the light the clouds scatter. A single gray afternoon tells you little. What counts is how much a roof produces over a whole year where you live, and the national lab's weather data can tell you that.

- Thick cloud can cut **100% of the direct beam** of sunlight, says the [Department of Energy](https://www.energy.gov/cmei/systems/solar-radiation-basics). The scattered, diffuse light still gets through.
- In our runs of NLR's PVWatts model, a 1 kW roof system produced about **1,088 kWh a year in Seattle**, **1,191 in Pittsburgh** and **1,755 in Phoenix**.
- The seasons swing output more than you might expect. In Seattle, **December output was 23% of July's**. In Phoenix it was 75%.
- Cold isn't the problem: solar cells **generally work best at low temperatures**.
- A battery stores power. It **doesn't make more** on a gray day.

## Why solar panels work on cloudy days, at less power

The Department of Energy's [solar radiation basics](https://www.energy.gov/cmei/systems/solar-radiation-basics) split sunlight into two parts. Direct beam radiation is light that reaches the ground without being scattered. Diffuse radiation is light that air molecules, water vapor, clouds, dust and pollutants have absorbed, scattered and reflected along the way. "The sum of the diffuse and direct solar radiation is called global solar radiation."

Clouds change the mix. The DOE says atmospheric conditions can reduce the direct beam by 10% on clear, dry days and by 100% on thick, cloudy days. On a heavily overcast day, the panels are working mostly or only on diffuse light, so output drops. It doesn't drop to zero.

You'll find confident percentages online for how much a cloudy day costs. They disagree with each other and we couldn't trace any of them to a primary source, so we've left them out. If you're deciding whether to buy, one day's weather tells you far less than the yearly total.

## What clouds and winter do over a year

The National Laboratory of the Rockies (NLR, formerly NREL) runs [PVWatts](https://pvwatts.nlr.gov/), a public online model that estimates a system's output for any address. Version 8 uses [2020 typical-year weather data](https://developer.nlr.gov/docs/solar/pvwatts/v8/) from the lab's National Solar Radiation Database, so cloudy days are already counted in its answer.

We ran PVWatts V8 on 2026-10-14 for three cities with the same inputs: a 1 kW system, standard modules, fixed roof mount, 20° tilt, facing due south, 14% system losses. The numbers are kilowatt-hours of AC output.

| City | A year | July | December | December as a share of July |
| --- | --- | --- | --- | --- |
| Seattle, WA | 1,088 | 143 | 33 | 23% |
| Pittsburgh, PA | 1,191 | 134 | 55 | 41% |
| Phoenix, AZ | 1,755 | 156 | 117 | 75% |

So do solar panels work on cloudy days in a place like Seattle? They do, just less. The Seattle system makes about 62% of what the same system makes in Phoenix, and Pittsburgh about 68%. That gap is real, and it means a cloudy-city system needs more panels for the same output, but it doesn't settle on its own whether solar makes sense for you.

The seasons make a big swing too. The DOE notes that the sun's rays "are far more slanted during the shorter days of the winter months," and that Denver gets nearly three times more solar energy in June than in December. In the Seattle run, July and August each produce more than four times what December does.

Your roof isn't our test roof, either. These are model estimates for one setup. Actual production varies with weather, soiling, shading and equipment, and a roof facing east or west, or shaded by trees, will come in lower. PVWatts lets you enter your own address, tilt, direction and a loss percentage, so run it for your house.

Tool: [How many panels do you need?](https://solarrealcost.com/does-solar-pay/tools/how-many-panels). From what you actually use, the panel wattage available, and the production factor where you live.

If you know your yearly electricity use, the number PVWatts gives you per kilowatt is what turns it into a system size. Our guide on [how many solar panels you need](https://solarrealcost.com/does-solar-pay/how-many-solar-panels-do-i-need) walks through that step.

## Do solar panels work in winter and cold?

Cold by itself doesn't hurt. The DOE's [performance basics](https://www.energy.gov/cmei/systems/solar-photovoltaic-performance-and-efficiency-basics) say "solar cells generally work best at low temperatures." Heat is what pulls their voltage down. Winter output falls because the days are short and the sun is low, not because the air is cold. Seattle sits much farther north than Phoenix, and its December number shrinks far more.

The DOE's [homeowner guide](https://www.energy.gov/cmei/systems/homeowners-guide-solar) says "solar panels are built to work in all climates," and that they usually perform best on south-facing roofs with a slope between 15 and 40 degrees.

## What a battery does on cloudy days, and what it doesn't

A battery doesn't make the panels produce more on a gray day. It stores energy they already made. The DOE's homeowner guide says storage helps you use solar power when it's needed most, "regardless of whether it's sunny outside at the time," and even when the power goes out.

Without a battery, most rooftop systems don't keep the lights on in a blackout, cloudy or not. Standard home inverters "disconnect themselves from the grid and shut down" during a long or larger-than-normal disruption, the DOE's [inverter basics](https://www.energy.gov/cmei/systems/solar-integration-inverters-and-grid-services-basics) explain. If backup matters to you, see [what a solar battery costs](https://solarrealcost.com/does-solar-pay/solar-battery-cost) and price it as its own line.

**Check the production number in any quote**
An installer's estimate for a cloudy region should look like a PVWatts run for your address, not like a Phoenix figure. If a proposal promises much more than PVWatts gives for the same size, direction and tilt, ask what weather data and losses it assumed.

## Bottom line: do solar panels work on cloudy days?

Yes. Clouds can remove the direct beam, but panels still run on diffuse light, and cold weather doesn't hurt them. The real cost of a cloudy, northern climate shows up in the yearly total: in our PVWatts runs, about 1,088 kWh per kW in Seattle against 1,755 in Phoenix, and October through March account for about 69% of that gap (our arithmetic on the PVWatts runs). Whether that pays off depends on your electricity price and the [cost to install](https://solarrealcost.com/does-solar-pay/install-solar-panels-cost), so start from your own address and check whether [solar panels are worth it](https://solarrealcost.com/does-solar-pay/are-solar-panels-worth-it-now) at your numbers.

Start the Solar Check: https://solarrealcost.com/check

## Common questions

### Do solar panels work on cloudy days?

Yes, but they produce less. Clouds can block all of the direct beam of sunlight, and what reaches the panels is the scattered, diffuse light. Even in Seattle's lowest month, a PVWatts run for a 1 kW roof system still shows about 33 kWh of output.

### Do solar panels work in winter?

They do, and solar cells generally work best at low temperatures. Winter output is lower because days are shorter and the sun is lower. In our PVWatts runs, December output was 23% of July's in Seattle, 41% in Pittsburgh and 75% in Phoenix.

### Is solar worth it in a cloudy city?

It depends on your price for electricity and what the system costs, not on clouds alone. In our PVWatts runs, the same 1 kW system produced about 1,088 kWh a year in Seattle, 1,191 in Pittsburgh and 1,755 in Phoenix. Run the numbers for your own address.

### Will a battery let my panels make power when it's cloudy?

No. A battery stores energy the panels already made so you can use it later. It does not add generation on a gray day.

## Sources

- [U.S. Department of Energy: Solar radiation basics](https://www.energy.gov/cmei/systems/solar-radiation-basics)
- [U.S. Department of Energy: Solar photovoltaic performance and efficiency basics](https://www.energy.gov/cmei/systems/solar-photovoltaic-performance-and-efficiency-basics)
- [National Laboratory of the Rockies: PVWatts calculator](https://pvwatts.nlr.gov/)
- [National Laboratory of the Rockies: PVWatts V8 API documentation](https://developer.nlr.gov/docs/solar/pvwatts/v8/)
- [U.S. Department of Energy: Solar integration, inverters and grid services basics](https://www.energy.gov/cmei/systems/solar-integration-inverters-and-grid-services-basics)
- [U.S. Department of Energy: Homeowner's guide to going solar](https://www.energy.gov/cmei/systems/homeowners-guide-solar)