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Possible to determine watts of panel when testing it?

I did a little test as a proof of concept. I plotted the volts and amps of a panel in a sweep of the load. The blue is the current. Notice the flat top portion which is Isc. Then current starts dropping. The yellow is the voltage of the panel which is rising till Voc is reached. Red is the multiplication of I and V. Where it peaks the Vmp can be found. This is an old 12v panel and it has a long extension cord connecting it to the tester. That added resistance is why 12.6V is lower than expected. It is neat to see and know those published curves come from somewhere. Tester is small enough that it can be placed into a small hand held device like you have seen for $50. This is what they are doing except a micro reads and calculates the data. and it can be done in microseconds. I hope to later show multiple regional peaks for arrays in shade.



PVcurve_14_00012vs.jpg
 
Must be old panel with very poor shunt resistance, or you have low illumination on panel where shunt leakage current starts to dominate.

Current is dropping off way too soon indicating low shunt resistance, at least in relation to a reasonably good illumination current generation.

Typical panel shows Vmp at about 0.81 to 0.85 Voc, and Imp about 0.92-0.95 x Isc near full sun illumination.

Panel leakage current typically starts to dominate below about 0.2 full sun illlumination.
 
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I was just looking to get a representative curve. Going back and looking at it I see the 100W panel was producing only 5W as it had gone into some shade. Vmp does shift some as intensity lowers. I took another one today and got up to 32W being an overcast day. Looking better at 14.8, the figures don't lie. There is 50 feet of very light extension cord and there is some voltage loss there. I have 9 of these panels removed from service two years ago. Three of them had to have bypass diodes replaced after a lightning strike. I was in a rush getting the system back up and running. They seemed to work and I moved on to grid tie panels. I need to check them before I put them up for sale.PVcurve32W14.8V2.2As.jpg
 
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