What is the “edge-of-cloud effect” and how can it cause a solar array issues?

What is it? Well, just as the cloud begins to cover the sun or when the sun is emerging from behind a cloud, there is a sudden burst of energy that produces more power than normal. This is caused by light refraction. Refraction can concentrate the sunlight while the edge of the shadow passes by. The result is a boost in module voltage output. On a day with bright blue skies and fair weather cumulus clouds, the effect is quite noticeable.

So how can you account for this increase in output. Common practice is to add 20% to 25% to the amperage rating of the solar controller. But many controllers today are the MPPT type. They track the arrays Maximum Power Point on its IV curve. As the edge of clouds start causing over-irradiatance. The MPP voltage starts to rise, so too, does the current. The MPPT controller then adjusts the voltage up to correct for this effect.

Take for example a Sunny Boy 5000 Watt grid-tie inverter. The lower the voltage of the array the better the efficiency. Of course the design of an array depends on the solar panels but you should never design around the highest voltage under standard conditions. In this case 480 VDC. Me, I would design around 350VDC to 400VDC under normal operating conditions. This would allow for the MPP to move around where it wants to.

Basically, solar controller can handle brief overages of current, but not voltage. The edge-of-cloud effect is only going to be a problem when it is very cold outside and passing clouds. You can use this sizing tool for SMA Sunny Boy Products. Also, solar panels have a Voc (open circuit) and a Vmp (max power). for a 12VDC panel Vmp may actually be 17VDC and Voc around 21VDC. Since the edge-of-cloud effect normally occurs when the inverter or controller is running you already have some extra wiggle room.

(I guess you could have a rolling black during the day and have your grid-tie inverter shut-off. But then you would also have to have record cold temperatures. But then it would be winter and you would have less transmitted light because it has to travel though more atmosphere at an oblique angle. Plus you would have to have perfectly clean panels. As you can see, not likely to happen any time soon.)