Performance
Do solar panels work on cloudy days?
Yes — solar panels generate electricity from diffuse light on cloudy days, typically producing 10-25% of their peak output. UK weather averages 60-70% cloudy days per year, but cloudy generation accumulates substantially: a 100 kW system in the UK generates 85,000-105,000 kWh/year despite cloud cover. Panels actually perform marginally better in cool cloudy conditions than in hot direct sun above 30°C.
Yes — solar panels work on cloudy days. They generate electricity from diffuse light, typically producing 10-25% of their peak rated output under heavy cloud cover, and 30-60% under thin/broken cloud. The UK averages 60-70% cloudy days per year, yet a 100 kW system in the Midlands still generates 90,000-95,000 kWh annually because cloudy generation accumulates substantially across thousands of daylight hours. Counterintuitively, panels perform marginally better in cool cloudy conditions than in hot direct sun above 30°C — silicon panels lose about 0.4% efficiency per °C above 25°C, so the cooler the panel, the higher its conversion ratio. UK climate, with its temperate temperatures, suits silicon PV better than hot Mediterranean climates.
How cloudy-day generation works
Sunlight has two components:
- Direct beam radiation — sunlight that reaches the panel without scattering. Strongest under clear skies.
- Diffuse radiation — sunlight scattered by clouds, particles, and atmospheric molecules. Reaches the panel from all directions of the sky.
Solar panels convert both. Under clear midday sun, direct beam dominates (around 75-85% of incident energy). Under heavy cloud, diffuse light dominates (close to 100%) but total irradiance is much lower.
UK reality: most days are partially cloudy. Generation is variable across the day as clouds pass.
Typical generation by sky conditions
For a 100 kW system at solar noon, summer:
| Sky condition | Generation rate |
|---|---|
| Clear blue sky | 80-100 kW |
| Light high cloud | 60-80 kW |
| Broken cumulus, sun visible | 40-70 kW (variable) |
| Overcast (thin) | 25-50 kW |
| Heavy overcast | 10-25 kW |
| Heavy rain | 5-15 kW |
| Snow on panels | 0-5 kW (snow blocks sunlight) |
Note that even heavy overcast still produces 10-25% of peak — useful electricity over a working day.
UK cloudy day annual impact
PVGIS data for an average UK Midlands site:
- Total annual irradiance: ~1,000 kWh/m² horizontal
- Of which clear-sky direct: ~600 kWh/m²
- Of which diffuse (from clouds, scatter): ~400 kWh/m²
Diffuse contributes 35-45% of total annual irradiance in the UK. Solar panels capture both, so cloudy-day generation is 35-45% of annual output. Lose that share and the system economics collapse — but the share is real and continuous.
Why UK climate suits solar better than you’d think
Three factors make UK solar perform better than naive expectations:
- Long summer days: Midsummer day-length is 16-17 hours in the UK Midlands, only 11-12 hours in Mediterranean Spain. Total summer generation is competitive.
- Cool panel temperatures: average UK summer panel temperature 35-45°C; Mediterranean 50-65°C. UK panels run closer to STC, achieving higher efficiency.
- Diffuse-rich climate: panels with anti-reflective glass and bifacial designs capture diffuse light efficiently. Modern Tier 1 panels are explicitly optimised for diffuse-heavy climates.
Net result: UK solar yields are 80-85% of Mediterranean yields per kW installed, despite the cloudier climate.
Worked example: cloudy week vs sunny week
Take a typical 100 kW Midlands install in May.
Sunny week (5 mostly-clear days):
- Daily generation: 450-520 kWh
- Weekly total: 2,300-2,500 kWh
Cloudy week (5 overcast days, occasional rain):
- Daily generation: 150-280 kWh
- Weekly total: 900-1,200 kWh
The cloudy week generates roughly half the sunny week. Both produce real, useful electricity.
Across a full year, UK weather averages out — generation varies year-to-year by ±5-8% but the long-run average is well established by 20+ years of UK solar data.
How to design for variable UK weather
- Don’t design assuming peak output continuously — instead, model annual kWh from PVGIS data.
- Specify inverters with high part-load efficiency (97% peak; >95% at 30% load). UK inverters spend most time at part-load.
- Use string inverters with optimisers, or microinverters, where partial shading from clouds and obstacles is variable.
- Don’t oversize DC dramatically beyond AC — clipping risk in summer; reduces capture under cloud.
- Ensure 25-year linear performance warranty includes diffuse radiation performance.
Common misconceptions about cloudy weather
“Solar panels need direct sun” — wrong. They work in any daylight.
“UK is too cloudy for solar to be economic” — wrong by data. Commercial solar in the UK has 5-8 year payback, equivalent to or better than southern European installs of similar age (because UK grid prices are higher).
“Panels stop completely when cloudy” — wrong. They continue at 10-25% of peak.
“You need batteries to make cloudy-day solar work” — wrong. Cloudy generation is still self-consumed in real time. Batteries help with timing, not with cloud per se.
Next steps
For a UK weather-modelled feasibility study, request a quote. See: UK cloudy weather detail, panel output, efficiency, cost guide, grants page.
Related questions
How efficient are commercial solar panels?
Commercial solar panels in 2026 typically achieve 20-23% efficiency at Standard Test Conditions, with leading Tier 1 modules (Trina Vertex S+, JA Solar Deep Blue, Longi Hi-MO 7) reaching 22.5-23%. Higher efficiency means more kW from less roof area, useful where space is tight. The system as a whole loses 8-15% to inverter, cabling, soiling, shading, and temperature — real-world AC output is typically 85-92% of DC nameplate.
How much energy does a commercial solar panel produce?
A typical 540 W commercial solar panel in the UK produces 480-580 kWh per year, with the variation driven by location (south coast vs Scottish highlands), orientation, pitch, and shading. South-facing 35-40 degree pitch in the Midlands is the benchmark — about 530 kWh/panel/year. The whole system produces 850-1,050 kWh per kW of nameplate annually.
How does UK cloudy weather affect commercial solar performance?
UK cloudy weather is fully accounted for in commercial solar yield calculations — the long-run average is 850-1,050 kWh/kW/year across the UK, with year-to-year variation typically ±5-8%. Cloudy days reduce instantaneous output but accumulate over the year. UK silicon PV achieves 80-85% of Mediterranean yields per kW because cooler panel temperatures partly offset higher cloud cover.