shade and pv...

alexis.marcil

Hello,

I have a simple question but cannot seem to find the answer anywhere... What is the efficiency of pv panels during cloudy day. I know it must be low, but still, if it's 10% for 10 hours, it amount to something at the end of the day!

thanks

Alexis

icarus

Re: shade and pv...

Anecdotally, my system that puts out ~10% of normal on a thin, overcast day. Real grey, rain, drops to almost nothing. Where it might put out ~16 amps, it puts out about 1.6.

Also keep in mind that is would be VERY unusual for a fixed mount system to get 10 hours of sun, except in the high latitudes in the summer. The sort of rule of thumb for calcs is somewhere between 4-6 hours of ideal sun, a couple more of some insolation. My system captures the sunrise 365 days a year, aimed perfectly for it in August, and it won't come up to meaningful numbers until a couple hours after sunrise. The angle is low, and the sun shines through more atmosphere.

Remember, you mileage may vary!

Tony

mike95490

Re: shade and pv...

It's so low, I don't think any bothers to write it down. More like 5% when cloudy. Some rainy days, there is not even enough power to start up the GT inverter. That's like <1% of a 4.5KW system.

Clouds mean generator run time.

icarus

Re: shade and pv...

There is also a difference between "shade" and overcast. Shadows from buildings, or limbs or wires has a huge effect on production. Overcast in various density drops production off, until some the sky is cloudy enough to drop it to near zero.

Partial shading of a string wired in series has a dramatic effect on output. Depending on how the panels are configured, a single telephone wire can drop the entire string to near zero. Arrays wired in parallel suffer less. A shade on one panel might drop the productin of that panel to near zero, but the entire string would only see the production of the one panel lost.

Tony

Windsun

Re: shade and pv...

The "efficiency" does not change. However the amount of light that the panel receives does go down, and that is what counts.

Shade from an object or tree cuts it down 90-97% depending on close and diffuse the shadow is.

niel

Re: shade and pv...

for the record, just because they are clouds does not mean it is not shading. it still is shading, but it just isn't as well defined and this can be from pollution in the air or even dirt on the pvs in addition to the more obvious wires, trees, planes, etc.

Mangas

Re: shade and pv...

Also, when siting a fixed array try set the field up to where the front panels do not shade the ones standing behind during winter/summer sun cycles.