Weather and Solar Yield: How Weather Affects Your PV System

Your solar system’s daily output is largely determined by the weather. But the relationship between weather and yield is more complex than “sun = power, clouds = no power.” This article explores all key weather factors in detail.

Direct vs. Diffuse Irradiance

Solar irradiance reaching your modules consists of two components:

• Direct irradiance: Sunlight arriving in a straight line from the sun. Maximum on clear days: ~1,000 W/m²
• Diffuse irradiance: Light scattered by clouds, atmosphere, and particles. On overcast days: 100-300 W/m²

Clear sky1.000 W/m²

Light clouds700 W/m²

Partly cloudy450 W/m²

Overcast200 W/m²

Heavy rain80 W/m²

Even on fully overcast days, significant diffuse irradiance reaches the modules. Modern solar cells are specifically optimised for diffuse light conditions.

Cloud Types and Their Impact

Cloud Type	Altitude	Output Reduction	Frequency (Germany)
Cirrus (thin, high)	6-12 km	10-20%	Common
Cumulus (fair weather)	1-6 km	40-60%	Very common in summer
Stratus (overcast)	0-2 km	60-80%	Common in autumn/winter
Cumulonimbus (storm)	1-15 km	80-95%	Occasional in summer
Altocumulus (medium)	2-6 km	30-50%	Common

Cloud Enhancement Effect

A remarkable phenomenon: when sunlight breaks through cloud edges, irradiance can briefly exceed 1,000 W/m² — sometimes reaching 1,200-1,400 W/m². This “cloud enhancement” or “lensing effect” occurs when clouds act as reflectors, focussing additional light onto the modules. These peaks last only seconds to minutes but can temporarily boost output beyond rated capacity.

Temperature: Why Cool Sunny Days Are Ideal

25°C (STC)400 Wp

35°C386 Wp

45°C372 Wp

55°C358 Wp

65°C344 Wp

75°C330 Wp

This explains why the most productive days are often cool, clear spring days (April/May) rather than the hottest summer days.

Rain: Short-Term Loss, Long-Term Benefit

Rain reduces output significantly during the event (80-95% loss). However, it provides a valuable self-cleaning effect:

• Dust, pollen, bird droppings and leaf debris are washed away
• Post-rain output can be 2-5% higher than before
• Regular rain reduces the need for manual cleaning

Snow: Total Block vs. Albedo Effect

• Snow-covered modules: zero output
• Tilted roofs (30°+): snow slides off within hours to days
• After clearing: the albedo effect from ground snow can boost output by 10-25% — reflected light from white surfaces increases total irradiance

Wind: Invisible Performance Booster

Wind cools module surfaces and reduces operating temperature:

Wind Speed	Temperature Reduction	Output Improvement
0 km/h (calm)	Reference	Reference
10 km/h	-5 to -8°C	+1.5-3%
20 km/h	-8 to -12°C	+3-4.5%
30 km/h	-10 to -15°C	+4-5.5%

Well-ventilated mounting systems with rear ventilation amplify this cooling effect.

Practical Tips

1. Check the forecast: Use the Solar Forecast to plan high-consumption activities on sunny days
2. Clean after dry spells: If it hasn’t rained for weeks, a gentle rinse can recover 2-5% yield
3. Monitor temperature: If your monitoring shows high module temperatures, ensure adequate ventilation
4. Winter planning: Don’t count on winter yield for self-sufficiency — a battery storage helps bridge cloudy periods

Check your daily yield forecast now — free and location-specific