Interesting story - most of you already know this, but I was dumb struck by the magnitude of the effect...

 

We had an array of 7 panels in series (360Wp each), running just over a year.

Yesterday finally installed an 8th panel we'd provisioned for - expecting a 14% increase in yield.

Monitoring the PV output this morning, seemingly it was producing extra but wasnt sure and waiting for midday peak - it started consistently dipping around 11am. "Weird, no clouds, what the ???" I thought.

It dawned on me - the newly installed panel has a small chimney that may be casting a shadow on the new panel.

But surely that can't be? The dip showed lower production than what the 7 panels on their own would produce!!! Surely with shadow on 8th panel I'd still produce 7 panels plus one additional "downgraded" panel???

Anyways, climbed on the roof, and there was indeed a small 8cm by 8cm shadow right at the bottom of the newly installed panel (total array size 8m x 2m, size of each panel 2m x 1m, so the small 8 x 8cm shadow really was comparatively tiny).

Long story short, I cut the chimney - no more shadow, and the PV output immediately jumped by almost 25%!!!! 

(See image below)

 

I understand any shadow will handicap the system somewhat, but didn't realize how much it would actually handicap the entire array! Wow, lesson learnt, if any part of your array gets shadow throughout the day then either get rid of that shadow, or split into 2 or more separate arrays to minimize the effect...

On 2021/05/09 at 3:11 PM, 87 Dream said:

so hence it's like having a string of 7 like these in series

that is incorrect. The diodes will bypass the shaded panel/s and the string will now operate at a lower voltage. The output from the unshaded panels is not affected. The problem is, that without a good shadow management the inverter will not find the correct mpp and as a result operate the string at the wrong voltage. Another problem is, that the voltage of the string might fall below the inverter minimum operating threshold. This is why long strings (if some shading occurs) are better than short ones.