Do Solar Panels Work on Cloudy Days? Real Production Numbers by Climate

The short answer: yes, solar panels produce power on cloudy days — typically 10% to 25% of what they'd generate under a clear sky, depending on how thick the cloud cover is. A light haze or thin overcast might only cost you 10-30% of output. A thick, dark rain cloud can knock production down to single digits. The panels never fully stop working; they just have far less usable light to convert.

The bigger question for homeowners in Seattle, Portland, Pittsburgh, or anywhere else known for gray skies isn't "do panels work" — it's "will a system sized for my roof actually cover my usage across a full year of real weather." That's a sizing question, not a physics question, and it has a concrete answer.

Last updated: 2026-07-13

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How Clouds Actually Affect Solar Production

Solar panels need light, not necessarily direct sunlight. Photovoltaic cells respond to photons hitting the cell — and clouds scatter those photons rather than blocking them entirely. This scattered light is called diffuse irradiance, as opposed to the direct beam irradiance you get on a clear day.

On a clear day, roughly 85-90% of the light reaching your panels is direct beam. On a cloudy day, that flips — most or all of the light is diffuse, bounced around by water droplets in the clouds before it reaches the ground. Diffuse light still generates electricity, just at a much lower intensity.

The reduction in output depends on cloud density:

• Thin, high clouds or haze: 10-30% reduction from clear-sky output
• Moderate overcast: 40-60% reduction
• Thick, dark storm clouds: 75-90% reduction

There's one counterintuitive wrinkle worth knowing: partly cloudy days with bright white clouds near the sun can occasionally produce brief spikes above clear-sky output, because sunlight reflects off the cloud edges and adds to the direct beam hitting your panels. It's a short-lived effect, but it's real and shows up in production monitoring data as odd spikes.

Real Production Numbers: Cloudy Cities vs. Sunny Cities

The most useful comparison isn't clear-day vs. cloudy-day — it's how a full year of a cloudy region's actual weather adds up compared to a sunny region. NREL's PVWatts data, which models production using decades of real weather history, gives a clear picture for a 10kW residential system:

Seattle, WA (cloudy ~226 days/year):

• Annual production: ~10,800-11,500 kWh
• Peak sun hours: ~3.4/day average

Phoenix, AZ (cloudy ~35 days/year):

• Annual production: ~17,500-18,500 kWh
• Peak sun hours: ~6.5/day average

Pittsburgh, PA (cloudy ~206 days/year):

• Annual production: ~11,800-12,600 kWh
• Peak sun hours: ~3.8/day average

Denver, CO (cloudy ~115 days/year):

• Annual production: ~15,600-16,400 kWh
• Peak sun hours: ~5.6/day average

Seattle produces roughly 60-65% of what Phoenix produces from an identically sized system. That's a real gap — but it's not a reason to skip solar. It's a reason to size the system correctly and set expectations around a properly designed quote, not a rule-of-thumb estimate.

Why Cloudy-Climate Solar Still Pencils Out

Two things offset the lower per-panel output in cloudy regions, and both matter more than most homeowners expect going in.

Electricity rates in cloudy states tend to run higher. Washington is an exception (cheap hydro power keeps rates low), but Oregon, Pennsylvania, Massachusetts, and most of the cloudy Northeast and Pacific Northwest pay well above the national average per kWh. A system that produces less energy but offsets electricity priced at 22-30 cents/kWh can still generate similar savings to a sunnier state charging 12-14 cents/kWh.

Cloudy climates are usually also mild climates. Seattle and Portland don't have the brutal air-conditioning loads that drive summer electric bills through the roof in Phoenix or Houston. Lower baseline usage means a system doesn't need to chase an enormous summer peak — it just needs to match a steadier, more moderate year-round load.

The practical takeaway: don't use a national average cost-per-watt figure or a generic "sun hours" chart to estimate your savings. Get a quote built on your specific roof, your specific utility rate, and your actual annual usage from your last 12 months of bills.

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Cloudy-Climate Solar FAQs

Does rain clean or damage panels?

Rain actually helps — it rinses off dust, pollen, and light debris, which is one small silver lining to frequent precipitation. Rain doesn't damage panels; they're rated for far harsher weather than typical rainfall.

Do I need extra panels to make up for cloudy weather?

Your installer's production estimate already accounts for your local weather history, so the "extra panels for cloudy weather" adjustment is already built into a properly sized quote. You don't need to ask for more beyond what the modeling recommends.

Is solar worth it at all in a place like Seattle or Pittsburgh?

Yes, for most homeowners — assuming a quote based on real local weather data and your actual utility rate shows a reasonable payback period, typically 7-12 years depending on local incentives and electricity costs. The physics work fine in cloudy climates; it's a math problem, not a feasibility problem.

How much does snow affect production compared to clouds?

Snow cover blocks nearly all production until it slides or melts off, which is a separate issue from cloud cover. A steeply pitched, dark-colored panel typically sheds snow within a day or two of the storm ending.

Key Takeaways

• Solar panels still generate power on cloudy days — typically 10-25% of clear-sky output, depending on cloud thickness
• Diffuse light (scattered by clouds) still produces electricity; panels don't need direct beam sunlight to work
• A cloudy city like Seattle produces roughly 60-65% of what a sunny city like Phoenix produces from an identical system size
• Higher electricity rates and milder cooling loads in most cloudy climates offset the lower per-panel output
• A legitimate quote is sized using your ZIP code's actual historical weather data, not a national sun-hours average — ask to see the monthly production breakdown
• If you're pairing solar with battery backup, a multi-day overcast stretch recharges more slowly — a power station with fast wall-outlet charging adds a buffer clouds alone can't drain

Related articles:

• Do Solar Panels Work in Winter? Real Production Numbers and What to Expect
• How to Use EnergySage to Compare Solar Quotes (and Spot the Red Flags)
• Solar Panel Cost Per Watt in 2026: What You'll Actually Pay