Posted by: nik | March 3, 2014

Edge-of-Cloud Effect

It’s March! The sun is getting higher in the sky, days are getting longer, and equinox is only a few weeks away. It’s that time of year when cabin fever sets in and spring just won’t come soon enough. But it is also the time of year when we see the highest power readings from our solar electric array. The nominal peak DC power rating of our whole system is 9.36 kilowatts . That means under “Standard Test Conditions” (1,000 W/m2 solar irradiance, 25°C PV module temperature and air mass of 1.5) the panels will be providing 9,360 Watts of power to the inverters.  The inverters are the electronic devices that convert this DC solar power to typical AC power to be used in the house or sold back to the utility. The inverters operate at about 98% efficient. There are also some losses due to shading, inefficiencies in the wiring and connections, soiling on the panels, less than ideal orientation, etc, etc. For design purposes we usually use a derating of about 75% of the system STC rating. So it is very rare that you will ever see the output of your solar PV system at or above the nominal DC rating. This is usually the time of year that it happens. The  operating temperaures are good (colder is better for the semiconductor effect) and the moisture level in the air is at a minimum. The following chart is a 10-minute snapshot of our power profile for yesterday at noon. The green line is solar power reading (updated every minute) and red line is our consumption.

The Edge-of-Cloud Effect at work

The Edge-of-Cloud Effect at work


You can see sustained solar readings over 9,000 Watts during this period and a spike up over 9,500! That’s  9,560 AC Watts! That’s 200 Watts above the DC rating of the array! The dip right in the middle of the chart is a small cloud passing in front of the array. As the cloud moves away this is when the highest reading happens. This is known as the “Edge-of-Cloud Effect.” The sunlight is being magnified as it passes through the moisture droplets in the edge of the cloud. The other conditions (temperature, sun angle, minimal shading, etc.) all combine to give the highest solar output reading of the year.

Happy March!



  1. Great post, Nik.
    Sadly, higher power doesn’t translate to more energy. Because the edge of cloud effect is a passing phenomenon, it doesn’t generate enough energy to be noticeable in our daily totals. Still pretty neat.

    • Good point. It’s probably more interesting to solar designers than consumers. We use at least 125% of peak rating for sizing all the electrical components. This is why!

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