Cloud cover vs solar output: what actually happens on overcast UK days
Cloudy doesn’t mean zero. Learn the difference between thin and thick overcast, why output often looks flatter (not broken), and how to benchmark your kWh using your postcode.
Cloudy day. Grey sky. Your solar app looks… underwhelming.
It is easy to jump to: “Something’s wrong with my system.”
In most cases, nothing is wrong at all.
If you want to sanity-check what “normal” looks like where you live, Find your postcode and benchmark against a realistic local baseline.
UK solar is not “sun or nothing”. A lot of UK generation comes from diffuse light, which still exists on overcast days. What changes under cloud is not just how much light you get, but what kind of light reaches your panels.
This guide explains:
- Why panels still generate on cloudy days (and why the curve looks different)
- Why some “overcast” days do surprisingly well, and some do terribly
- What “normal” variability looks like in real UK conditions
- How to benchmark your output using kWh (kilowatt-hours, your energy total, see Glossary) over time, not one disappointing hour
Along the way, we’ll use postcode examples from across the UK, from TR1 (Truro) to AB10 (Aberdeen), without turning this into a physics lecture.
Quick Answer: Should You Worry on Cloudy Days?
- Cloudy does not mean “zero”. Your panels still generate from diffuse light.
- Overcast days often produce a flatter, lower curve rather than a sharp midday peak.
- Thin, bright overcast can generate surprisingly well. Thick, dark overcast can be genuinely poor.
- What matters is your monthly and yearly kWh, not whether a single day looked disappointing.
- If your longer-term kWh is broadly in line with a realistic estimate for your postcode, your system is almost certainly fine.
- Do not judge performance by peak kW (instant power, see Glossary) on one cloudy afternoon.
If you want the practical benchmark:
- Look at your kWh total for the last month (or better: the last 12 months)
- Compare against a realistic estimate for your location and setup using the SolarByPostcode calculator
- If you are broadly in the right ballpark, ignore noisy day-to-day cloud swings
Assumptions and variability
- We assume your system is basically healthy (no persistent inverter faults, string failures, or monitoring dropouts).
- We assume you are comparing like-for-like periods (season matters a lot, so “January vs June” is not a fair test).
- Cloud impact varies most with cloud thickness, sun angle (time of year), and your roof direction and shading.
- Short-term graphs are noisy by nature in the UK, especially on broken-cloud days.
- For how SBP estimates local output and savings, see Data sources and methodology.
Cloudy Weather Changes the Light Mix
On a clear day, most of your generation comes from direct sunlight.
On an overcast day, most of it comes from diffuse sky light.
That difference is why cloudy-day output often looks “wrong” if you expect a sharp midday peak.
What diffuse light means in real life
Diffuse light is basically sunlight that has been scattered by the sky and clouds. You do not get sharp shadows, but you still get usable energy.
This is why it is completely normal for an overcast day in BN1 (Brighton) to produce “something reasonable” while still looking disappointing compared to a clear day. It is also why some UK arrays produce best when the sky is “bright grey” rather than blazing blue.
Why Cloudy Output Looks “Flatter”
Most people expect solar to be a bell curve: low in the morning, spike at midday, low again.
Cloud cover often turns that into a lower, flatter hump.
This matters because many people assess performance by the peak. Under cloud, the peak is the wrong thing to look at.
If you are trying to judge whether your system is healthy, focus on kWh over days and months, not the shape of a single day’s curve.
Not All “Overcast” Is the Same
Overcast is not a single condition. Two grey skies can behave very differently.
- Thin / bright overcast: the sky looks white, the day feels bright, you get decent diffuse light.
- Thick / dark overcast: the day feels dim, lights come on indoors, output can be genuinely weak.
Those percentages are not promises. They are a mental model to stop you treating all cloud as identical.
In practice, you will see this difference clearly if you compare a “bright grey” day in CB1 (Cambridge) to a “dark grey” day in G1 (Glasgow) in the same season.
The Cloud Effects That Confuse People Most
1. “It’s bright outside, but my solar is low”
This happens when the brightness is coming from general sky glow, not direct sun. Diffuse light can feel bright to humans but still deliver less energy than a clear day.
That is normal, especially in hazy, high-cloud conditions you often see around big urban areas like SW1A (Westminster).
2. “My power jumps up and down every few minutes”
That is the classic broken-cloud day. Sun breaks through, then gets blocked, repeatedly.
It is common in Atlantic-influenced areas like TR1 (Truro), PL1 (Plymouth) and CF10 (Cardiff).
The graph looks chaotic, but over the full day the kWh can still be decent.
3. “It’s cloudy and yet I saw a sudden spike higher than expected”
This can happen near the edge of a cloud when the cloud acts like a bright reflector, briefly boosting irradiance. It is real, it is usually short, and it is not something you should chase as a performance target.
Cloud, Postcode, and Why UK Comparisons Are Tricky
Cloud patterns vary a lot across the UK.
Two people with the same system size can have very different cloudy-day expectations depending on:
- regional cloudiness
- coastal vs inland weather
- latitude (winter sun angles are harsh in the north)
- local haze and pollution (often thin, persistent high cloud effects)
That is why comparing a friend in BN1 (Brighton) to a cousin in BT1 (Belfast) is rarely useful.
Instead, compare your output to a realistic local benchmark using your postcode: SolarByPostcode.
A broken-cloud day can look very different in different places:
- SO14 (Southampton) might show long bright periods with short dips.
- M1 (Manchester) might show a lower, flatter curve all day.
- EH1 (Edinburgh) might show similar cloud but with a lower winter ceiling due to sun angle.
None of those patterns imply a fault. They are weather signatures.
How to Tell “Normal Cloud Loss” from a Real Problem
Cloud creates noise. Faults create patterns.
Here is the practical way to separate them.
Step 1: Stop judging by a single day
Pick a longer window:
- last 7 days (minimum)
- last month (better)
- last 12 months (best)
Cloud variability is huge day-to-day. Faults show up as sustained underperformance across many days.
Step 2: Use kWh, not peak kW
A cloudy day can still deliver meaningful energy even if the peak looks unimpressive.
If you have not read it yet, the guide on why peaks are a trap is here:
Why Your Solar Panels Will Never Hit Their Rated Output (And Why That’s Completely Normal)
Step 3: Compare against a postcode-based estimate
Use your postcode and your rough setup details to get a realistic expectation: SolarByPostcode
If you are consistently far below a realistic estimate, then it is worth investigating:
- shading (trees, chimneys, nearby buildings)
- inverter issues
- string failures (one part of the array underperforming)
- monitoring faults (data reporting wrong even if production is fine)
If you are roughly in range, cloud is just cloud.
If your longer-term totals look fine but a few summer days underperform on very hot afternoons, that is often temperature-related rather than cloud-related:
Temperature losses in UK solar panels: how heat quietly eats your kWh
If you are seeing a slow year-on-year drift (not just weather noise), degradation is a better explanation than “more cloudy days”:
Solar panel degradation in the UK: what you really lose over 25 years
Three Myths About Cloudy UK Solar
Myth 1: “Overcast means solar is basically pointless”
Wrong. UK solar works precisely because panels still generate in diffuse light.
Overcast reduces output, but it does not switch it off.
Myth 2: “If my app is low on a cloudy day, the panels must be dirty”
Dirt can matter, but cloud is usually the bigger driver on any given day.
If you want to check for soiling, compare similar clear days across months (or year-on-year), not a sunny day against a cloudy day.
Myth 3: “My friend’s system does better on cloudy days, so mine is faulty”
Unless your friend is in the same region with a similar roof, this comparison is close to meaningless.
A system in NR1 (Norwich) can see a very different “typical overcast” experience to a system in NE1 (Newcastle) or AB10 (Aberdeen).
Practical Tips: How to Get the Most Value Under UK Cloud
1. Shift flexible loads into the middle of the day
On overcast days you often still have a usable midday plateau. If you can shift loads (dishwasher, washing machine, immersion heater) into late morning or early afternoon, you increase self-use.
This is often more valuable than chasing the perfect clear-day peak.
2. If you are planning a battery, do not size it from a “perfect day”
Cloudy days are part of the UK reality. Size based on realistic averages, not on a single glorious June day.
3. If your roof is east-west, cloud can be less dramatic than you think
With east-west arrays, you often get a broader production window. Under diffuse light, that can look surprisingly steady.
If you are curious how your roof direction changes expectations, our Aspect Penalty Map shows the relative impact across roof directions.
Frequently Asked Questions
Do solar panels work on cloudy days in the UK?
Yes. They still generate from diffuse light. Output is usually lower, but not zero. Thin, bright overcast can perform much better than people expect.
Why does my solar graph look flat when it is overcast?
Overcast reduces direct sunlight and increases diffuse light. Diffuse light tends to produce a lower, steadier curve rather than a sharp midday peak.
Is it normal for output to jump up and down on cloudy days?
Yes. Broken cloud causes rapid changes in sunlight as the sun repeatedly appears and disappears behind clouds. That “spiky” pattern is a weather signature.
How do I know if low output is cloud or a fault?
Use kWh over longer windows (weeks and months) and compare against a realistic postcode-based estimate. Cloud causes noisy day-to-day variation. Faults tend to cause sustained, consistent underperformance.
Next reads
- Why solar panels never hit their rated output (and why that is normal)
- Solar monitoring: how to spot underperformance early (without obsessing)
- Shading and solar panels: when a single tree really does matter
- Temperature losses in UK solar panels: how heat quietly eats your kWh
- Winter solar generation in the UK: why batteries can’t fix seasonality
Bottom Line
Cloud is not a verdict on your system. It is just the UK doing UK weather.
- Cloud changes the light mix, not just the brightness
- Overcast often creates a flatter curve, which is normal
- “Bright overcast” and “dark overcast” behave very differently
- Diagnose performance with kWh over time, and benchmark with your postcode