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Posts: 21
Reply with quote  #1 
I've read that it is good practice to add up to 33% more solar panels than you inverter's rating. This will give you higher efficiency without damaging your equipment.

I have recently purchased a DC clamp meter to check what current my two solar panel strings are generating. When my inverter was giving it's maximum rated output it appeared that the second string current was being significantly restricted (say by 50%). Does anyone know if this is how inverters operate to limit output?
As well (and apart) from the above I noticed one string consistently had approx 20% less current than the other, this is reasonable as the panel strings are a bit different. The string with the highest current was showing significantly less PV.   Does this all make sense??


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Posts: 3,053
Reply with quote  #2 
Have you talked to the inverter manufacturer? This seems like a question for them.

My THEORY is that when you have two or more arrays in parallel the higher voltage string will overpower the other. When the inverter has to limit output, the "weaker" string will be reduced first.

Admittedly this is a THEORY and I have no hard evidence to back it up. I've observed it with batteries, and with a PV-alternator combo, but not with PV-PV. But it makes sense to me.

Is there any way you can reconfigure your system so that the strings are more balanced?

The only other way I can see to correct it would be to put the dissimilar strings on separate inverters ($$$).

Rick may have a solution.

Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

Posts: 21
Reply with quote  #3 
The manufacturer doesn't seem to respond to questions about their product.
I agree I could balance up the strings better.
I am thinking of adding more panels to the weaker string to boost the systems output during the winter months. I am considering installing an isolator (at ground access level) to open the link between two panels and closing a second isolator to incorporate three additional panels into the weaker string. I already have the isolators as I was going to get a second inverter installed on my system, but due to changes by the regulator that is no longer possible. The additional panels are already installed on the roof. I have additional heat sinks and temperature controlled fans installed on my inverter to keep its temperature reasonable. I also monitor temperature around my device.1-PB102014a.jpg 


Posts: 5
Reply with quote  #4 
Why not wire the extra panels in the week string and forget the switching?

Posts: 21
Reply with quote  #5 
I did that months ago, I have two strings of 12 panels. So far its working out OK.

Posts: 116
Reply with quote  #6 
Just a thought. Try disconnecting string 1, measure the power output, reconnect string 1, disconnect string 2, measure the power output of string 2. Are they the same assuming other conditions are the same? If not then one string is definitely weaker than the other. Perhaps there is a weak panel. Maybe there is shading or something blocking one panel a little that could limit the output current.

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