PV output question

[Lee Dodge]

Glen-

A couple of days after a snowstorm here in Colorado, I noticed that my 14 panel (Sunpower 225 W each with Sunpower 3000 W inverter) grid-tied system was only putting out about 250 W when it should have been putting out about 2500 W based on previous experience. I went out to clear the snow off the panels, and discovered that each panel was covered on only the bottom 15% or 20%, the rest having melted. As it melted the snow slid down to collect on the bottom lip of the panel. So 15% or 20% shading was causing a 90% loss of power. This experience reminded me that solar cells are diodes, and they must act like low resistance paths when they are not illuminated, degrading the other cells. My shading was a different geometry than yours, but it makes me think that shading of some of the solar cells may have more than a linear effect in degrading the performance of the overall system.

[gfbankston]

Well, it should not make any difference what panel is hooked where if there operating current is close, which these are. I have 'balanced' the system to
get the use of those older BP panels...the funny thing is that the string with the BP panels has the higher output.

As they say " if it was easy, everyone would be doing it"

I wanted to give an update on this 'problem'.

I moved 9 of the CS5P-230 panels north, more out of the shade. I have checked the inverter output today, at peak sun angle. The inverter showed 2450 W at
one point in time, and was bouncing around 2300-2400 W output at the time.

Going back and using the PTC ratings of these panels I came up with 2770 W. Now, multiply that time 94% for inverter inefficiency give a little more than 2600 W.
So, 2450 W peak is not that far from the 2600 W I should expect. I will add another 400W worth of panels when I can to use the capability of the inverter.

Shading was my main problem. The shade will completely be gone in a couple months...

[ggunn]

I suppose I could stick pins in the USE-2 cable and measure the voltage on each one.

Don't do that. It probably wouldn't tell you anything useful and you would be opening a path to the conductor for water to get in. And then there's the safety issue; while the pin to pin voltage would not be all that much for a single module, the pin to ground potential could be considerable, depending on where in the string you would be taking your measurement.