I’m pleased to share relevant solar content to you, my passion and profession here at OneRoof Energy. This is the second of a three-part series covering how different aspects of weather impact solar PV system performance. Today we’ll discuss the impacts of cloud cover on your production – Chris K.
You might think that a solar PV system will perform better when it’s sunny outside and not so much when it’s cloudy. That’s generally correct, but it can still produce under certain cloudy conditions. Just as an example, Germany – which has less annual average sunlight penetration than the United States – is the largest solar market in the world.
This picture will give you an idea (provided by NREL) of the amount of sunlight intensity (or average energy flux from the sun as measured in kWh/m2/year) of Germany as compared to the United States. From this, it shows that the average yearly sunlight intensity is higher in all states, including Alaska and Hawaii, than Germany. There are other things at play, including the latitude of the locations, but cloud cover is a bigger factor on sunlight intensity.
To give you the Cliff Notes version: California has an average of about 50 days of rainfall per year – implying days with significant cloud cover. Germany has an average of about 115 days per year, double that of California! So even though there is significant cloud cover, solar PV systems are still performing.
Let’s first identify some definitions on types of cloud cover:
Light-cloudy sky: conditions with less than 30% cloud cover. This sky is combined with sun in most cases
Partly-cloudy sky: conditions between 30 and 70 % cloud cover. This sky can be combined with sun in some cases
Cloudy sky: conditions with more than 70% cloud cover. This sky normally excludes the sun
Overcast sky: conditions with completely closed cloud cover (100%). No sun penetration here.
In general, when the sun is shining directly overhead in an open sky (no cloud cover), the sunlight intensity is at its max. Under light- or partly-cloudy skies, sunlight is absorbed and scattered by the clouds. More of the light that comes through is diffused (scattered, not direct) resulting in less sunlight intensity at any particular spot, like on your solar PV system. Cloudy sky conditions can reduce sunlight intensity by over 70 percent and overcast conditions could result in more than 85 percent reduction. These two conditions make a big difference in how your system will produce energy.
One interesting point on cloud cover, much like how you can get sunburned even when it’s overcast – your system can still produce energy. Ultraviolet light produced by the sun is what causes sunburns on skin, while the visible light spectrum is what primarily generates electricity in solar PV. Under partly cloudy conditions, visible and ultraviolet sunlight can penetrate clouds, but they do so by being reflected or refracted, meaning that the sunlight is scattered.
How does ORE compensate for cloudy days when we calculate your average production?
We actually use forecasts as one tool. Some factors that impact sunlight intensity can be precisely forecasted on a day-to-day basis. While cloud cover isn’t as predictable as other conditions because of all of the different ways clouds are created, we can predict periods of cloudy conditions (think “May Gray” or “June Gloom” in Southern California). General patterns of cloudiness can be forecasted and can be lumped into longer term prediction models to give you a better estimate of performance.
So to answer the question more precisely, performance models factor in cloud cover as part of annual averages for a particular weather station. What really matters is how much energy solar PV systems produce in the long run. This allows ORE to account for short term cloud cover by averaging into the long-term production and give homeowners a more accurate over-all production estimate.
Check back for our next post in the weather series on how temperature and wind impacts solar PV systems and their performance.
