Solar panels continue to generate electricity even when the sky is overcast and rain is falling, though performance is noticeably diminished compared to a bright, sunny day. Power production persists because the fundamental science of solar energy conversion allows panels to work even when direct sunlight is blocked by clouds. The power generated is still fed into the home or the electricity grid, ensuring continuous energy contribution from the photovoltaic array.
Understanding Diffuse Light and Panel Function
Photovoltaic (PV) cells generate electricity by converting light particles, called photons, into a flow of electrons—a process known as the photovoltaic effect. PV cells are designed to respond to the full spectrum of light, meaning power production depends entirely on the presence of light, not just intense, direct rays from the sun.
When the sun is shining clearly, panels receive direct sunlight that travels straight to the Earth’s surface. During cloudy or rainy conditions, panels rely on diffuse light. Diffuse light is sunlight that has been scattered, reflected, or refracted by atmospheric elements like clouds, water droplets, or dust.
This scattered light reaches the solar panel from multiple directions. Modern solar cells, particularly those using crystalline silicon technology, are highly capable of capturing this lower-intensity, multidirectional light. Although the light reaching the panels is less intense, meaning fewer photons hit the silicon, the conversion mechanism remains active.
Measuring Energy Output on Cloudy Days
The presence of heavy cloud cover or rain directly correlates with a reduction in the electrical output of a photovoltaic system. This drop occurs because atmospheric moisture and clouds absorb and scatter a large portion of the incoming solar radiation. Under conditions of a light drizzle with moderate cloud cover, a solar array may still operate at 40% to 60% of its peak performance.
When the weather becomes heavily overcast, such as during a heavy downpour with dark storm clouds, the output drops significantly. Energy production often falls into the range of 10% to 25% of the maximum capacity achieved on a clear day. The degree of reduction depends on the thickness and type of cloud cover, with dense, low-level clouds causing the most substantial efficiency loss.
The design of the solar installation, including the inverter technology used, can help mitigate some of this performance loss. Advanced components, such as microinverters, maximize the power harvest from each individual panel. This is particularly beneficial in low-light conditions or when partial shading occurs, ensuring that if one panel performs poorly due to a dense cloud patch, the output of the other panels is not dragged down.
How Rain Improves Panel Efficiency
While the clouds accompanying rain reduce instantaneous power production, the rainfall itself provides a distinct, long-term benefit to the solar array. This advantage stems from the cleaning action of the water on the panel’s surface. Over time, solar panels accumulate “soiling,” which includes dust, pollen, bird droppings, and industrial pollutants.
These layers of debris physically block light from reaching the photovoltaic cells, reducing the panel’s ability to absorb both direct and diffuse sunlight. Even a thin layer of grime can lead to a notable decrease in efficiency, sometimes by a few percentage points. Rainfall acts as a natural cleaning agent, washing away loose particles and restoring the panel’s glass surface to a clearer state.
The restorative effect of the cleaning is realized once the rain stops and the sun returns, allowing the panels to operate at a higher light transmission rate. Studies have shown that this natural cleaning process can boost energy production by 2% to 5% after the weather clears, recovering efficiency lost to surface contamination. For the rain to be most effective, panels are usually installed with a minimum tilt angle, allowing the water to flow off and carry the debris away rather than pooling on the surface.