Solar Panels and Clouds

For a given solar panel, the output will largely be determined by Sun-Earth geometry and the prevailing weather conditions.  The effect of clouds can be dramatic, a patch of low cloud passing between the Sun and the panel on a clear day results in an instantaneous drop in the current supplied by the panel.  Low, dense cloud which persists for several days can reduce the output of a solar panel by more than 80%, even during the summer months.   The two graphs show the effect of cloud cover, they are based on observations made a few days apart in May 2008, the first set of reading were made on a clear sunny day, the second were taken during a period of low cloud which persisted for three days.  In both cases, the load on the panel was a lead acid battery which was almost fully discharged at the start of day.  On the sunny day, the battery was fully charged by 15:30, on the cloudy day, the charge was no more than 25% of the battery's capacity (crude estimate).

The first graph shows the current delivered to the battery between 06: and 18:00:

The second shows the charge supplied to the battery (in mA hours):

The "clear" and "clouds" represent the extreme cases, most days consisting of a mix of blue sky and passing clouds.

On days when the sky state consists of a few, scattered or broken clouds there will be significant fluctuations in irradiance within a short space of time (and hence output) as can be seen in the graph below:

At the time these measurements were recorded, scattered cloud at an estimated height of 2 - 4,000 feet.

The Sky over Southern England

The pie chart show the distribution of sky states over southern England over a year:

Solar devices and the human body generally don't make too much distinction between a clear blue sky and one with a few clouds in it, but 50% of the time (mainly winter), clouds have a significant impact on the performance of solar devices and the desire of most Brits to be somewhere further south.

What Clouds Do

The effect of clouds is to reduce the amount of the Sun's radiant energy transmitted to the Earth's surface.  Part of the energy is reflected upwards and part of it is absorbed within the cloud itself.  This is illustrated in the diagram below:

The cloud also changes the nature of the irradiance.  The cloud acts as a diffuser, when the Sun is obscured by a cloud, the irradiance is not only attenuated, but becomes diffuse.

The proportion of energy transmitted is a function (amongst other things) of the type of cloud, it's height and thickness, the nature and extent of the cover and the time of day.  All of which conspire to make it difficult, if not impossible to model the process.

Clouds and Seasons

It can be inferred from the diagram above, the the greater the zenith angle, the lower the transmission through the cloud will be.  As the zenith angle increases, the absorption will increase at the mass of water that the beam passes through also increases, also reflection will increase.  In winter zenith angles are much larger than in summer.  The graph below was derived from the Electric Solar Bucket, it shows the clear sky attenuation during winter and summer:

It shows that effect of clouds during winter is much greater than during the summer months.

Precipitation

Rain and snow have a marked impact on the performance of solar devices.  Rain forms a dense layer of water droplets between the Sun and the Earth's surface. A snow shower effects panel performance to an even greater degree than rain. An added complication is that snow will adhere to the panel surface.  As snow is highly reflective, it can remain in place for several days preventing the panel from functioning.

Generalisations

However, there are a couple of generalisations based on the mass of water that the Sun's rays must pass through.  The effect of high level cloudHowever, there are a couple of generalisations based on the mass of water that the Sun's rays must pass through.  The effect of high level cloud can be small, whilst, thick, low cloud can reduce panel performance to less than 10% of its clear sky level.  As with most things solar, transmission on cloudy days is greater around solar noon when the zenith angle is low, than it is around dawn and dusk when the rays pass through a the cloud at an angle.